docs(build): establish build/dist structure with archive pattern

- Remove **/dist from global gitignore (allows tracking blessed releases)
- Add packages/cli/.gitignore to ignore its ephemeral dist/ (Rollup output)
- Create docs/build/build-dist-structure.md documenting archive pattern
- Update docs/README.md to link to new structure document

Build/dist pattern:
- build/ (gitignored) = workspace + archive of historical builds
  - build/tmp/ = Current build intermediates
  - build/cache/ = Download caches
  - build/archive/ = Historical builds with timestamps/configs
- dist/ (tracked) = Blessed canonical releases that ship

Benefits: Experimentation, comparison, rollback, history tracking

Author: jdalton

.gitignore

@@ -21,7 +21,6 @@ Thumbs.db
 **/.cache
 **/coverage
 **/.type-coverage
-**/dist
 **/build
 **/node_modules
 .build-checkpoints
@@ -30,13 +29,8 @@ Thumbs.db
 **/*.tmp
 *.tsbuildinfo
 
-# Build artifacts (inside build/ only).
-**/build/target/
-**/build/pkg/
-**/build/out/
-**/build/cmake/
-**/build/_deps/
-**/build/wasm/
+# Build artifacts: build/ = gitignored (workspace + archive), dist/ = tracked (blessed releases).
+# Exception: packages/cli/dist/ is Rollup output (ephemeral), gitignored by packages/cli/.gitignore.
 
 # Yarn PnP files (tests may create these)
 /.yarn

docs/README.md

@@ -23,6 +23,7 @@
 - [Unified binary design](architecture/unified-binary.md)
 
 ### Build System
+- [Build/dist structure and archiving](build/build-dist-structure.md)
 - [Build process overview](build/build-process.md)
 - [Build quick start](build/build-quick-start.md)
 - [Build system summary](build/build-system-summary.md)

packages/cli/.gitignore

@@ -0,0 +1,2 @@
+# Rollup bundled output (ephemeral build artifact, not a blessed release).
+dist/