Feat v0.2.9 builtin extension retirement

Stop auto-seeding 6 chat-centric extensions on startup. Move them to
standard packages with js/ layout and "requires": ["chat"] metadata.
Delete SeedBuiltinPackages, seed tests, Dockerfile COPY, and
extensions/builtin/ directory.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

CHANGELOG.md

@@ -2,6 +2,29 @@
 
 All notable changes to Switchboard Core are documented here.
 
+## v0.2.9 — Builtin Extension Retirement
+
+### Changed
+
+- **6 builtin extensions → standard packages**: csv-table, diff-viewer,
+  js-sandbox, katex-renderer, mermaid-renderer, and regex-tester moved from
+  `extensions/builtin/` to `packages/` with standard `js/` layout. Each
+  manifest now declares `"requires": ["chat"]` and `"type": "extension"`.
+  Built via `build.sh` like all other packages — no auto-install.
+
+### Removed
+
+- **`SeedBuiltinPackages` seeder**: Deleted `seed_packages.go` (function,
+  `builtinManifest` struct, `buildFullManifest` helper) and the startup call
+  in `main.go`. Extensions are no longer auto-seeded into the DB.
+- **`extensions/builtin/` directory**: Removed from repo and Dockerfile COPY.
+- **Seed tests**: Removed 8 `TestSeed_*` functions and `makeSeedDir` helper
+  from `extension_test.go` (~220 lines).
+- **Stale comments**: Removed `SeedBuiltinPackages` references from
+  `package_iface.go` and `trigger_sync.go`.
+
+---
+
 ## v0.2.8 — Team Admin Settings Audit (Pass 1)
 
 ### Removed