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.

ARCHITECTURE.md

@@ -1561,10 +1561,11 @@ graph TD
   - feature shipping
   - MCP operations
   - live issue debugging
-- `.claude/agents/` provides specialized review workers:
+- `agents/` provides specialized review workers (shipped as the `ccam` plugin's agents):
   - backend reviewer
   - frontend reviewer
   - MCP reviewer
+  - qc-local, senior-gate-reviewer (the ship-feature-lane pipeline's QC and publish gate)
 - `plugins/` provides distributable plugin marketplace (see [Plugin Marketplace](#plugin-marketplace)):
   - ccam-analytics (session reports, cost analysis, usage trends, productivity scoring)
   - ccam-productivity (standups, weekly reports, sprint summaries, workflow optimization)