feat: add Avian coding assistant support

Add Avian as a supported AI coding tool with skill and command
generation. Follows the same adapter pattern as Claude Code and Crush
with nested opsx command directories and YAML frontmatter.

Author: avianion

docs/supported-tools.md

@@ -24,6 +24,7 @@ You can enable expanded workflows (`new`, `continue`, `ff`, `verify`, `sync`, `b
 | Amazon Q Developer (`amazon-q`) | `.amazonq/skills/openspec-*/SKILL.md` | `.amazonq/prompts/opsx-<id>.md` |
 | Antigravity (`antigravity`) | `.agent/skills/openspec-*/SKILL.md` | `.agent/workflows/opsx-<id>.md` |
 | Auggie (`auggie`) | `.augment/skills/openspec-*/SKILL.md` | `.augment/commands/opsx-<id>.md` |
+| Avian (`avian`) | `.avian/skills/openspec-*/SKILL.md` | `.avian/commands/opsx/<id>.md` |
 | Claude Code (`claude`) | `.claude/skills/openspec-*/SKILL.md` | `.claude/commands/opsx/<id>.md` |
 | Cline (`cline`) | `.cline/skills/openspec-*/SKILL.md` | `.clinerules/workflows/opsx-<id>.md` |
 | CodeBuddy (`codebuddy`) | `.codebuddy/skills/openspec-*/SKILL.md` | `.codebuddy/commands/opsx/<id>.md` |
@@ -68,7 +69,7 @@ openspec init --tools none
 openspec init --profile core
 ```
 
-**Available tool IDs (`--tools`):** `amazon-q`, `antigravity`, `auggie`, `claude`, `cline`, `codex`, `codebuddy`, `continue`, `costrict`, `crush`, `cursor`, `factory`, `gemini`, `github-copilot`, `iflow`, `kilocode`, `kiro`, `opencode`, `pi`, `qoder`, `qwen`, `roocode`, `trae`, `windsurf`
+**Available tool IDs (`--tools`):** `amazon-q`, `antigravity`, `auggie`, `avian`, `claude`, `cline`, `codex`, `codebuddy`, `continue`, `costrict`, `crush`, `cursor`, `factory`, `gemini`, `github-copilot`, `iflow`, `kilocode`, `kiro`, `opencode`, `pi`, `qoder`, `qwen`, `roocode`, `trae`, `windsurf`
 
 ## Workflow-Dependent Installation
 

src/core/command-generation/adapters/avian.ts

@@ -0,0 +1,34 @@
+/**
+ * Avian Command Adapter
+ *
+ * Formats commands for Avian following its frontmatter specification.
+ */
+
+import path from 'path';
+import type { CommandContent, ToolCommandAdapter } from '../types.js';
+
+/**
+ * Avian adapter for command generation.
+ * File path: .avian/commands/opsx/<id>.md
+ * Frontmatter: name, description, category, tags
+ */
+export const avianAdapter: ToolCommandAdapter = {
+  toolId: 'avian',
+
+  getFilePath(commandId: string): string {
+    return path.join('.avian', 'commands', 'opsx', `${commandId}.md`);
+  },
+
+  formatFile(content: CommandContent): string {
+    const tagsStr = content.tags.join(', ');
+    return `---
+name: ${content.name}
+description: ${content.description}
+category: ${content.category}
+tags: [${tagsStr}]
+---
+
+${content.body}
+`;
+  },
+};

src/core/command-generation/adapters/index.ts

@@ -7,6 +7,7 @@
 export { amazonQAdapter } from './amazon-q.js';
 export { antigravityAdapter } from './antigravity.js';
 export { auggieAdapter } from './auggie.js';
+export { avianAdapter } from './avian.js';
 export { claudeAdapter } from './claude.js';
 export { clineAdapter } from './cline.js';
 export { codexAdapter } from './codex.js';

src/core/command-generation/registry.ts

@@ -9,6 +9,7 @@ import type { ToolCommandAdapter } from './types.js';
 import { amazonQAdapter } from './adapters/amazon-q.js';
 import { antigravityAdapter } from './adapters/antigravity.js';
 import { auggieAdapter } from './adapters/auggie.js';
+import { avianAdapter } from './adapters/avian.js';
 import { claudeAdapter } from './adapters/claude.js';
 import { clineAdapter } from './adapters/cline.js';
 import { codexAdapter } from './adapters/codex.js';
@@ -41,6 +42,7 @@ export class CommandAdapterRegistry {
     CommandAdapterRegistry.register(amazonQAdapter);
     CommandAdapterRegistry.register(antigravityAdapter);
     CommandAdapterRegistry.register(auggieAdapter);
+    CommandAdapterRegistry.register(avianAdapter);
     CommandAdapterRegistry.register(claudeAdapter);
     CommandAdapterRegistry.register(clineAdapter);
     CommandAdapterRegistry.register(codexAdapter);

src/core/config.ts

@@ -21,6 +21,7 @@ export const AI_TOOLS: AIToolOption[] = [
   { name: 'Amazon Q Developer', value: 'amazon-q', available: true, successLabel: 'Amazon Q Developer', skillsDir: '.amazonq' },
   { name: 'Antigravity', value: 'antigravity', available: true, successLabel: 'Antigravity', skillsDir: '.agent' },
   { name: 'Auggie (Augment CLI)', value: 'auggie', available: true, successLabel: 'Auggie', skillsDir: '.augment' },
+  { name: 'Avian', value: 'avian', available: true, successLabel: 'Avian', skillsDir: '.avian' },
   { name: 'Claude Code', value: 'claude', available: true, successLabel: 'Claude Code', skillsDir: '.claude' },
   { name: 'Cline', value: 'cline', available: true, successLabel: 'Cline', skillsDir: '.cline' },
   { name: 'Codex', value: 'codex', available: true, successLabel: 'Codex', skillsDir: '.codex' },

test/core/command-generation/adapters.test.ts

@@ -4,6 +4,7 @@ import path from 'path';
 import { amazonQAdapter } from '../../../src/core/command-generation/adapters/amazon-q.js';
 import { antigravityAdapter } from '../../../src/core/command-generation/adapters/antigravity.js';
 import { auggieAdapter } from '../../../src/core/command-generation/adapters/auggie.js';
+import { avianAdapter } from '../../../src/core/command-generation/adapters/avian.js';
 import { claudeAdapter } from '../../../src/core/command-generation/adapters/claude.js';
 import { clineAdapter } from '../../../src/core/command-generation/adapters/cline.js';
 import { codexAdapter } from '../../../src/core/command-generation/adapters/codex.js';
@@ -183,6 +184,39 @@ describe('command-generation/adapters', () => {
     });
   });
 
+  describe('avianAdapter', () => {
+    it('should have correct toolId', () => {
+      expect(avianAdapter.toolId).toBe('avian');
+    });
+
+    it('should generate correct file path with nested opsx folder', () => {
+      const filePath = avianAdapter.getFilePath('explore');
+      expect(filePath).toBe(path.join('.avian', 'commands', 'opsx', 'explore.md'));
+    });
+
+    it('should generate correct file paths for different commands', () => {
+      expect(avianAdapter.getFilePath('new')).toBe(path.join('.avian', 'commands', 'opsx', 'new.md'));
+      expect(avianAdapter.getFilePath('bulk-archive')).toBe(path.join('.avian', 'commands', 'opsx', 'bulk-archive.md'));
+    });
+
+    it('should format file with name, description, category, and tags', () => {
+      const output = avianAdapter.formatFile(sampleContent);
+      expect(output).toContain('---\n');
+      expect(output).toContain('name: OpenSpec Explore');
+      expect(output).toContain('description: Enter explore mode for thinking');
+      expect(output).toContain('category: Workflow');
+      expect(output).toContain('tags: [workflow, explore, experimental]');
+      expect(output).toContain('---\n\n');
+      expect(output).toContain('This is the command body.\n\nWith multiple lines.');
+    });
+
+    it('should handle empty tags', () => {
+      const contentNoTags: CommandContent = { ...sampleContent, tags: [] };
+      const output = avianAdapter.formatFile(contentNoTags);
+      expect(output).toContain('tags: []');
+    });
+  });
+
   describe('clineAdapter', () => {
     it('should have correct toolId', () => {
       expect(clineAdapter.toolId).toBe('cline');
@@ -606,7 +640,7 @@ describe('command-generation/adapters', () => {
     it('All adapters use path.join for paths', () => {
       // Verify all adapters produce valid paths
       const adapters = [
-        amazonQAdapter, antigravityAdapter, auggieAdapter, clineAdapter,
+        amazonQAdapter, antigravityAdapter, auggieAdapter, avianAdapter, clineAdapter,
         codexAdapter, codebuddyAdapter, continueAdapter, costrictAdapter,
         crushAdapter, factoryAdapter, geminiAdapter, githubCopilotAdapter,
         iflowAdapter, kilocodeAdapter, opencodeAdapter, piAdapter, qoderAdapter,