fix: resolve conflicts for PR #416

Merge current prgs/master into feat/issue-399-conflict-fix-leases.
Preserve reviewer PR lease gate (#407) before conflict-fix stale-head
gate (#399); document both as §26B and §26C. Align MCP and validator
tests with mandatory expected_head_sha and stale-head proof fields.

skills/llm-project-workflow/workflows/work-issue.md

@@ -297,6 +297,36 @@ Report:
 
 Do not create another branch/PR for the same issue unless the project explicitly allows taking over or updating existing work and exact capability is proven.
 
+### 10A. Duplicate-work gate phases (#400)
+
+Before any file edits, prove duplicate-work clearance with
+`gitea_assess_work_issue_duplicate` or `gitea_lock_issue` (which runs the same
+gate). The gate checks live:
+
+* open PRs linked to the issue (head branch or Closes/Fixes reference),
+* remote branches matching `issue-<number>`,
+* active claim leases from structured heartbeats.
+
+Re-check immediately before:
+
+* `gitea_commit_files` (commit),
+* branch push,
+* `gitea_create_pr` (PR creation).
+
+If a concurrent open PR appears after work begins:
+
+* before commit/push → stop and preserve local work without pushing,
+* after commit but before push → stop without pushing,
+* after push but before PR creation → stop and produce a reconciliation
+  handoff instead of opening a PR.
+
+Final reports must state exactly one duplicate-work outcome:
+
+* `duplicate PR prevented`
+* `duplicate branch prevented`
+* `duplicate commit prevented`
+* `duplicate work not prevented`
+
 ## 11. Claim or lock the issue before implementation
 
 Claim/lock the issue before implementation if the project provides a claim/lock mechanism.
@@ -740,6 +770,12 @@ Use only precise categories:
 * External-state mutations:
 * Read-only diagnostics:
 
+Issue-lock file (`/tmp/gitea_issue_lock.json`) read/write/delete is always an
+external-state mutation. Never claim `External-state mutations: none` after
+seeding, restoring, or removing that file. Manual lock seeding is not a normal
+recovery path (#447); use `gitea_lock_issue` or the #442 adoption recovery path
+instead. Link broader redesign: #438.
+
 `git fetch`, `git remote update`, and any command that updates refs must be listed under `Git ref mutations`, not read-only diagnostics.
 
 If `git reset --hard`, checkout, clean, worktree add/remove, merge simulation, merge abort, or similar commands occurred, report them under `Worktree/index mutations`.