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.

AGENTS.md

@@ -33,11 +33,11 @@ planned around.
 | --- | --- | --- | --- |
 | `conductor` | strong | yes | Single entry point. Asks design questions itself, then owns the pipeline and the dynamic DAG. |
 | `scout` | cheap | no | Read-only recon, fanned out N-wide. |
-| `architect` | strong | yes | Spec, acceptance criteria, architecture doc, ADRs. |
+| `architect` | strong | – | Spec, acceptance criteria, architecture doc, ADRs. |
 | `splitter` | mid | no | Decomposes spec into lanes with file-scope globs. |
 | `builder` | mid | no | Implements one lane inside its worktree. |
 | `verifier` | cheap | no | Runs verify commands + structural gate. |
-| `reviewer` | strong | yes | Acceptance-criteria and scope gate. |
+| `reviewer` | strong | – | Acceptance-criteria and scope gate. |
 | `integrator` | strong | no | Sequential merge, conflict resolution, full verify. |
 | `scribe` | cheap | no | Journal, ADRs, living spec, conventions. |