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.

ARCHITECTURE.md

@@ -1541,7 +1541,7 @@ graph TD
 
     CLAUDE --> C_MEM["CLAUDE.md"]
     CLAUDE --> C_RULES[".claude/rules/*"]
-    CLAUDE --> C_SKILLS[".claude/skills/*"]
+    CLAUDE --> C_SKILLS["skills/*<br/>ccam plugin skills"]
     CLAUDE --> C_PLUGINS["plugins/<br/>10 plugins, 53 skills"]
 
     CODEX --> X_MEM["AGENTS.md"]
@@ -1556,7 +1556,7 @@ graph TD
 
 - `CLAUDE.md` defines always-on project working agreements.
 - `.claude/rules/` adds path-scoped guidance by file area.
-- `.claude/skills/` provides reusable workflows:
+- `skills/` provides reusable workflows, shipped as the `ccam` plugin's own skills (`/ccam:<name>`, bare `/<name>` alias):
   - onboarding
   - feature shipping
   - MCP operations