feat: guarded merger lease adoption flow (#536)

Replace manual in-process _SESSION_LEASE seeding with an auditable
gitea_adopt_merger_pr_lease tool that posts durable adoption proof and
records sanctioned session provenance. Mutation gates now reject bare
record_session_lease calls without provenance from acquire/adopt/heartbeat.

Closes #536

docs/llm-workflow-runbooks.md

@@ -641,6 +641,33 @@ loop and do **not** substitute WebFetch/Playwright/manual base64.
 - **Prompt:** `Use any eligible merger profile to merge PR #N if checks pass and
   it is mergeable. Confirm with "MERGE PR N". Do not force-merge.`
 
+#### Merger lease adoption (#536)
+
+When review and merge run in **separate sessions**, the merger must **not**
+manually seed `reviewer_pr_lease._SESSION_LEASE` or equivalent in-process state.
+That ad hoc pattern was used incidentally for PR #493 and PR #421; it is not
+canonical proof and is rejected by mutation gates.
+
+**Canonical merger handoff:**
+
+1. Confirm a reviewer session holds an active PR lease and posted **APPROVED**
+   at the current live head (`gitea_get_pr_review_feedback`).
+2. In a clean merger worktree under `branches/`, call
+   `gitea_adopt_merger_pr_lease` with `worktree`, `expected_head_sha`, and
+   optional `issue_number`.
+3. The tool posts durable adoption proof on the PR thread (`<!-- mcp-review-lease-adoption:v1 -->`)
+   recording actor, profiles, adopted-from session/comment, adoption reason, and
+   timestamp, then records sanctioned in-session provenance.
+4. Call `gitea_merge_pr` with the same pinned `expected_head_sha`.
+
+**Forbidden:** Python one-liners or scripts that call `record_session_lease()`
+without provenance from `gitea_acquire_reviewer_pr_lease`,
+`gitea_adopt_merger_pr_lease`, or `gitea_heartbeat_reviewer_pr_lease`.
+
+**Same-session review+merge:** the reviewer session may use
+`gitea_acquire_reviewer_pr_lease` directly; adoption is only for cross-session
+merger handoff.
+
 ### Close the issue after merge / Reconciliation
 
 - **Profile:** issue-manager or merger.

docs/wiki/MCP-Tools.md

@@ -24,6 +24,8 @@
 - `gitea_dry_run_pr_review` — validation-phase review mechanics.
 - `gitea_mark_final_review_decision` — mark validation complete.
 - `gitea_submit_pr_review` / `gitea_review_pr` — gated live review.
+- `gitea_acquire_reviewer_pr_lease` / `gitea_heartbeat_reviewer_pr_lease` — per-PR reviewer lease (#407).
+- `gitea_adopt_merger_pr_lease` — guarded cross-session merger lease adoption (#536).
 - `gitea_merge_pr` — gated merge (only merge path).
 
 ## Read tools