feat(roles): add the read-only pr_reviewer role end-to-end

A global, read-only PR reviewer agent (pr-reviewer-1) that reviews inbound
external/fork PRs and posts one change-request. Wired end-to-end:

- identity: Role.PR_REVIEWER + agent + ROLE_LEVEL (QA-peer) + REVIEWER_ROLES
- lifecycle: CLAIM_RULES + ROLE_TEAM_RULES + a dedicated claim_pr_review /
  post_pr_review verb pair (distinct from QA's) + the pr_review_done action and
  its in_progress->completed transition; give_me_work / i_am_idle gain the role
- role_config: a read-only RoleConfig (allows_write=False)
- journaling: ALL_CELLS read tier so it can read internal intent like QA
- tracing: post_pr_review requires a learning entry; claim_pr_review is waived
- seeds presentation + factory prompt layer + builtin tools + the agentrole
  enum migration (037) + regenerated verb/lifecycle artifacts

Read-only at /app like QA/auditor; default-off — nothing dispatches review work
until external_pr_enabled. Foundation + role-config + enum suites green; ruff +
mypy clean; orchestrator boots.

panel/lib/lifecycle.json

@@ -17,6 +17,9 @@
     "main_pm": [
       "pending"
     ],
+    "pr_reviewer": [
+      "pending"
+    ],
     "product_owner": [],
     "qa": [
       "awaiting_qa"
@@ -33,6 +36,19 @@
       "pre_side_effects": [],
       "side_effects": []
     },
+    {
+      "allowed_roles": [
+        "pr_reviewer"
+      ],
+      "composes": [
+        "claim",
+        "start"
+      ],
+      "description": "Claim an inbound external-PR review task and start work. pending -> claimed -> in_progress.",
+      "name": "claim_pr_review",
+      "pre_side_effects": [],
+      "side_effects": []
+    },
     {
       "allowed_roles": [
         "qa"
@@ -114,6 +130,7 @@
         "developer",
         "documenter",
         "main_pm",
+        "pr_reviewer",
         "qa"
       ],
       "composes": [],
@@ -157,6 +174,7 @@
         "documenter",
         "head_marketing",
         "main_pm",
+        "pr_reviewer",
         "product_owner",
         "prompter",
         "qa",
@@ -234,6 +252,18 @@
       "pre_side_effects": [],
       "side_effects": []
     },
+    {
+      "allowed_roles": [
+        "pr_reviewer"
+      ],
+      "composes": [
+        "pr_review_done"
+      ],
+      "description": "Post one complete change-request to the external PR and finish the review task. in_progress -> completed.",
+      "name": "post_pr_review",
+      "pre_side_effects": [],
+      "side_effects": []
+    },
     {
       "allowed_roles": [
         "cell_pm"
@@ -528,6 +558,14 @@
       "source": "in_progress",
       "target": "cancelled"
     },
+    {
+      "action": "pr_review_done",
+      "roles": [
+        "pr_reviewer"
+      ],
+      "source": "in_progress",
+      "target": "completed"
+    },
     {
       "action": "pause",
       "roles": null,