refactor(agents): hide architect and reviewer from /agent picker

Enforce single-entry-point: only conductor is user-invocable now.
architect and reviewer remain fully functional but are only reachable
as subagents that conductor delegates to internally, matching the
same internal-only treatment already given to scout/splitter/builder/
verifier/integrator/scribe.

.github/agents/architect.agent.md

@@ -4,7 +4,7 @@ description: Turns decisions into a checkable spec, acceptance criteria, archite
 model: claude-opus-5
 tools: [read, search, edit, execute, agent, context7]
 agents: [Scout]
-user-invocable: true
+user-invocable: false
 ---
 
 # Architect

.github/agents/reviewer.agent.md

@@ -3,7 +3,7 @@ name: Reviewer
 description: Reviews completed lanes against every acceptance criterion and scope rule, then returns explicit approval or rejection.
 model: claude-opus-5
 tools: [read, search, execute]
-user-invocable: true
+user-invocable: false
 ---
 
 # Reviewer