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.

.claude/rules/file-headers.md

@@ -3,4 +3,4 @@
 - Every applicable source file (`.js/.ts/.tsx/.cjs/.mjs/.py/.sh/.css` — excluding `node_modules/`, `dist/`, `data/`, minified/vendored, snapshots) MUST start with the header comment: a truthful file overview plus the exact line `@author Nguyễn Ngọc Trí Vĩ <vinnt@smartgift.vn>`.
 - Creating a new applicable file → write the header before any code (after the shebang in scripts).
 - Editing a file that lacks the header → add it in the same change; if the edit changes the file's purpose, update the overview.
-- Formats and the repo-wide audit script live in `.claude/skills/file-headers/` (`bash .claude/skills/file-headers/scripts/check-headers.sh` must exit 0).
+- Formats and the repo-wide audit script live in `skills/file-headers/` (`bash skills/file-headers/scripts/check-headers.sh` must exit 0).