[1/5] Aitools: state tracking, manifest source, directory rename

[simonfaltum]

PR Stack

This is part of the aitools feature-complete initiative. PRs should be reviewed and merged in order.

1. [1/5] State + release discovery + directory rename (this PR)
2. [2/5] Install writes state + interactive agent selection ([2/5] Aitools: install writes state, interactive agent selection, idempotent install #4811)
3. [3/5] Update + uninstall + version commands ([3/5] Aitools: update, uninstall, and version commands #4812)
4. [4/5] List improvements + command restructuring + flags ([4/5] Aitools: list command, flat structure, --skills/--agents/--include-experimental flags #4813)
5. [5/5] Project scope (--project/--global) (pending)

Manifest v2 PR: (pending in databricks-agent-skills)

Why

The aitools skills installer has no state tracking. There's no record of what was installed, at what version, or when. This blocks every lifecycle command (update, uninstall, version). The manifest fetching logic is also hardcoded with no abstraction for testing.

Changes

Before: Skills install to ~/.databricks/agent-skills/ with no state file. FetchManifest is inlined in installer.go with a hardcoded ref and no way to mock it in tests.

Now:

• InstallState type with LoadState/SaveState (atomic write via temp+rename) for tracking installed skills, release ref, and timestamps
• ManifestSource interface with GitHubManifestSource, separating manifest fetching from install logic. Enables test mocking.
• FetchLatestRelease to discover newest release from GitHub API. Falls back to pinned ref on network failure.
• v2 fields on SkillMeta (Experimental, Description, MinCLIVer) with omitempty for backward compat with v1 manifests
• Canonical directory renamed from .databricks/agent-skills to .databricks/aitools/skills
• Backward-compat check in HasDatabricksSkillsInstalled for the old path

No behavior changes to existing commands except the directory path.

Test plan

• LoadState returns nil, nil for nonexistent file
• SaveState + LoadState roundtrip preserves all fields including optional ones
• SaveState creates nested directories, writes trailing newline
• LoadState returns error for corrupt JSON
• GlobalSkillsDir resolves correctly
• ProjectSkillsDir returns ErrNotImplemented
• HasDatabricksSkillsInstalled detects legacy path
• All existing aitools tests still pass

[github-actions]

Suggested reviewers

Based on git history of the changed files, these people are best suited to review:

• @arsenyinfo -- recent work in experimental/aitools/lib/installer/, experimental/aitools/lib/agents/
• @pietern -- recent work in experimental/aitools/lib/installer/, experimental/aitools/lib/agents/

Confidence: high

Eligible reviewers

Based on CODEOWNERS, these people or teams could also review:

@databricks/eng-apps-devex, @lennartkats-db

_{Suggestions based on git history of 6 changed files (3 scored). See CODEOWNERS for path-specific ownership rules.}