feat(plugin): ship the agents from a single root agents/ directory

Move .claude/agents/ and server/data/agent-templates/ship-feature-lane/
into one agents/ at the repo root — which IS the plugin root, so all five
agents now ship with the ccam plugin as ccam:<name>, the same way skills
already do.

The two pipeline agents were byte-identical copies in both places; that
duplication is gone. `ccam lanes agents install` now reads them straight
from agents/, so the per-lane copy in <lane>/.claude/agents/ still gives
the bare `subagent_type: qc-local` the ship-feature-lane skill prefers.
Skill and docs record both names: bare inside a lane, ccam: as fallback.

INSTALL.md

@@ -238,7 +238,7 @@ This repository includes extension packs for both Claude Code and Codex.
   - `CLAUDE.md`
   - `.claude/rules/`
   - `skills/` (shipped as the `ccam` plugin's skills)
-  - `.claude/agents/`
+  - `agents/` (shipped as the `ccam` plugin's agents)
 - Codex project packs live under `.codex/`:
   - `AGENTS.md`
   - `.codex/rules/`