feat(plugin): ship the repo's skills as ccam plugin skills

The repo root already IS the `ccam` plugin root (`"source": "./"`), so the
conventional `skills/` dir at the root is the plugin's skills dir — while
`.claude/skills/` only loaded for a session whose cwd was inside this
checkout. That is useless for a lane working in another repo, which is why
`ccam skills install` existed at all.

Move all seven skills to `skills/`: they now reach every plugin user as
`/ccam:<name>` (bare `/<name>` as an alias). `ccam skills install` stays as
the fallback for a plain checkout without the plugin, sourcing from the new
path.

Agents deliberately do NOT move. A plugin agent is addressed as
`ccam:qc-local`, which would break the `subagent_type: qc-local` references
in ship-feature-lane; `ccam lanes agents install` keeps writing them into the
lane's own `.claude/agents/`, the highest-priority agent source. MCP was
already plugin-shipped via `plugins/ccam/.mcp.json`.

Mechanical fallout: ~115 source files carry a doc-comment pointer to
`.claude/skills/file-headers/`, repointed to `skills/file-headers/`. The
generic Claude Code config-surface examples in openapi/cc-config (`.claude/
skills/code-reviewer`) describe any user's layout and are left alone.

Also fixes the two file-headers scripts, whose `ROOT=` climbed four levels
from `.claude/skills/file-headers/scripts/` and would have escaped the repo
from the new depth — it scanned sibling projects until corrected to three.

npm run test:server (1160 pass), test:client (422 pass), mcp:typecheck,
mcp:build + mcp:check-build all green.

skills/ship-feature/references/feature-checklist.md

@@ -0,0 +1,22 @@
+# Feature Checklist
+
+- Scope
+  - Problem and success criteria are explicit.
+  - Impacted layers identified (server/client/mcp/docs/scripts).
+
+- Implementation
+  - Input validation and error handling are explicit.
+  - Existing behavior preserved where not in scope.
+  - Safety controls preserved.
+
+- Verification
+  - Backend: `npm run test:server` when backend changed.
+  - Frontend: `npm run test:client` when UI changed.
+  - MCP: `npm run mcp:typecheck` + `npm run mcp:build` when MCP changed.
+
+- Documentation
+  - `README.md`, `ARCHITECTURE.md`, `SETUP.md`, `INSTALL.md`, `mcp/README.md` updated as needed.
+  - Commands in docs match `package.json`.
+
+- Delivery
+  - Known risks and unrun checks are clearly stated.