feat(author-workflow): enforce exact issue lock before branch/commit/push/PR (Issue #204)

Recreation of the #204 work from closed PR #205 (invalid provenance), rebuilt
cleanly on master under the prgs author identity with no PR #203 content:

- Add gitea_lock_issue MCP tool: locks exactly one issue to its branch name,
  fails closed on branch/issue-number mismatch and on issues already tied to
  an open PR (by head branch or Closes/Fixes reference).
- gitea_create_pr now requires the issue lock: head must match the locked
  branch, title/body must contain Closes/Fixes #<locked issue> exactly, and
  ambiguous references (equivalent / related / same as) are rejected.
- scripts/worktree-start refuses to create an issue-linked worktree unless the
  lock file exists and matches the requested branch.
- assess_controller_handoff rejects handoffs whose selected issue / opened PR
  fields carry multiple numbers or fuzzy equivalence wording.
- Tests: TestIssueLocking (lock + create_pr gates), handoff exact-reference
  tests, worktree-start lock coverage.

Closes #204

Co-Authored-By: Claude Fable 5 <[email protected]>

mcp_server.py

@@ -13,6 +13,7 @@ Configuration (mcp_config.json):
         "env": {}
     }
 """
+import json
 import os
 import re
 import sys
@@ -174,6 +175,11 @@ import issue_duplicate_gate  # noqa: E402
 import role_session_router  # noqa: E402
 
 
+# Fail-closed exact-issue-lock file (#204): written by gitea_lock_issue,
+# consumed by gitea_create_pr and scripts/worktree-start.
+ISSUE_LOCK_FILE = "/tmp/gitea_issue_lock.json"
+
+
 def _reveal_endpoints() -> bool:
     """Admin/debug opt-in (#120): include endpoint URLs and token source
     names in tool output. Off by default so normal LLM-facing responses
@@ -505,6 +511,84 @@ def gitea_create_issue(
     return _with_optional_url({"number": data["number"]}, data.get("html_url"))
 
 
+@mcp.tool()
+def gitea_lock_issue(
+    issue_number: int,
+    branch_name: str,
+    remote: str = "dadeschools",
+    host: str | None = None,
+    org: str | None = None,
+    repo: str | None = None,
+) -> dict:
+    """Lock exactly one Gitea issue and its branch name to ensure durable tracking.
+
+    Args:
+        issue_number: The tracking issue number.
+        branch_name: The branch name (must match (fix|feat|docs|chore)/issue-<issue_number>-<desc>).
+        remote: Known instance — 'dadeschools' or 'prgs'.
+        host: Override Gitea host.
+        org: Override Org.
+        repo: Override Repo.
+    """
+    # 1. Enforce branch name includes issue number
+    expected_pattern = f"issue-{issue_number}"
+    if expected_pattern not in branch_name:
+        raise ValueError(
+            f"Branch name '{branch_name}' must contain locked issue pattern '{expected_pattern}' (fail closed)"
+        )
+
+    # 2. Check if the issue already has an open PR (reuse protection)
+    h, o, r = _resolve(remote, host, org, repo)
+    auth = _auth(h)
+    url = f"{repo_api_url(h, o, r)}/pulls?state=open"
+
+    try:
+        prs = api_get_all(url, auth)
+    except Exception as e:
+        raise RuntimeError(f"Could not list open PRs to verify issue lock: {e}")
+
+    for pr in prs:
+        pr_head = pr.get("head", {}).get("ref", "")
+        pr_title = pr.get("title", "")
+        pr_body = pr.get("body", "")
+
+        if expected_pattern in pr_head:
+            raise ValueError(
+                f"Issue #{issue_number} is already tied to an open PR (PR #{pr.get('number')}, branch '{pr_head}') (fail closed)"
+            )
+
+        patterns = [
+            f"closes #{issue_number}",
+            f"fixes #{issue_number}",
+        ]
+        text_to_check = f"{pr_title} {pr_body}".lower()
+        if any(p in text_to_check for p in patterns):
+            raise ValueError(
+                f"Issue #{issue_number} is already tied to an open PR (PR #{pr.get('number')}) via Closes/Fixes reference (fail closed)"
+            )
+
+    data = {
+        "issue_number": issue_number,
+        "branch_name": branch_name,
+        "remote": remote,
+        "org": o,
+        "repo": r,
+    }
+
+    try:
+        with open(ISSUE_LOCK_FILE, "w", encoding="utf-8") as f:
+            json.dump(data, f)
+    except Exception as e:
+        raise RuntimeError(f"Could not write issue lock file: {e}")
+
+    return {
+        "success": True,
+        "message": f"Successfully locked issue #{issue_number} to branch '{branch_name}' (fail-closed check complete).",
+        "issue_number": issue_number,
+        "branch_name": branch_name,
+    }
+
+
 @mcp.tool()
 def gitea_create_pr(
     title: str,
@@ -532,6 +616,41 @@ def gitea_create_pr(
         dict with 'number' of the created PR ('url' only with the reveal opt-in).
     """
     h, o, r = _resolve(remote, host, org, repo)
+
+    # ── Issue Lock Validation (Issue #194 / #196) ──
+    if not os.path.exists(ISSUE_LOCK_FILE):
+        raise RuntimeError("Issue lock is missing (fail closed). Call gitea_lock_issue first.")
+
+    try:
+        with open(ISSUE_LOCK_FILE, "r", encoding="utf-8") as f:
+            lock_data = json.load(f)
+    except Exception as e:
+        raise RuntimeError(f"Could not read issue lock file: {e} (fail closed)")
+
+    locked_issue = lock_data.get("issue_number")
+    locked_branch = lock_data.get("branch_name")
+
+    if head != locked_branch:
+        raise ValueError(
+            f"PR head branch '{head}' does not match locked branch '{locked_branch}' (fail closed)"
+        )
+
+    # Check for forbidden terms anywhere in title/body
+    forbidden_terms = ["equivalent", "related", "same as"]
+    text_to_check = f"{title} {body}".lower()
+    for term in forbidden_terms:
+        if term in text_to_check:
+            raise ValueError(
+                f"PR title or body contains forbidden term '{term}' (fail closed)"
+            )
+
+    # Ensure Closes #<locked_issue> or Fixes #<locked_issue> is present exactly
+    closes_pattern = re.compile(rf"\b(closes|fixes)\s+#{locked_issue}\b", re.IGNORECASE)
+    if not closes_pattern.search(text_to_check):
+        raise ValueError(
+            f"PR title or body must contain 'Closes #{locked_issue}' or 'Fixes #{locked_issue}' exactly to ensure durable tracking (fail closed)"
+        )
+
     auth = _auth(h)
     url = f"{repo_api_url(h, o, r)}/pulls"
     payload = {"title": title, "body": body, "head": head, "base": base}