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.

bin/ccam.js

@@ -1522,7 +1522,7 @@ async function cmdLanesAdd(args) {
 }
 
 /**
- * `ccam skills install` — copy this checkout's `.claude/skills/ship-feature-lane/`
+ * `ccam skills install` — copy this checkout's `skills/ship-feature-lane/`
  * into `~/.claude/skills/ship-feature-lane/`, so `/ship-feature-lane` is
  * discoverable from a session running inside any lane's own working
  * directory, not just inside this repo. Pure filesystem action; does not
@@ -2560,7 +2560,7 @@ const COMMAND_GROUPS = [
       [
         "skills install",
         "",
-        "Install .claude/skills/ship-feature-lane/ into ~/.claude/skills/ so /ship-feature-lane works from any lane",
+        "Install skills/ship-feature-lane/ into ~/.claude/skills/ so /ship-feature-lane works from any lane",
       ],
       [
         "lock status|acquire|release",