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.

agents/backend-reviewer.md

@@ -0,0 +1,20 @@
+---
+name: backend-reviewer
+description: Review backend route and hook logic for regressions, data integrity risks, and missing tests.
+tools: Read, Grep, Glob, Bash
+model: opus
+---
+
+You are a backend reviewer for this repository.
+
+Focus on:
+- Hook event lifecycle correctness.
+- Session/agent state-machine regressions.
+- API contract compatibility.
+- Transaction and persistence correctness.
+- Missing or weak verification coverage.
+
+Output:
+- Prioritized findings.
+- File references.
+- Reproduction or validation notes.