fix: auto-detect issue number from git branch in cycle_retro

When --issue is not provided, cycle_retro.py now parses the current
git branch name (e.g. kimi/issue-492) to extract the issue number.
This prevents all retro records from having issue=null when the
caller omits the flag.

Fixes #492

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

scripts/cycle_retro.py

@@ -44,6 +44,8 @@ from __future__ import annotations
 
 import argparse
 import json
+import re
+import subprocess
 import sys
 from datetime import datetime, timezone
 from pathlib import Path
@@ -97,6 +99,27 @@ def _epoch_tag(now: datetime | None = None) -> tuple[str, dict]:
     return tag, parts
 
 
+BRANCH_ISSUE_RE = re.compile(r"issue-(\d+)")
+
+
+def _detect_issue_from_branch() -> int | None:
+    """Try to extract an issue number from the current git branch name.
+
+    Matches branch patterns like ``kimi/issue-492`` or ``fix/issue-17``.
+    Returns ``None`` when not on a matching branch or git is unavailable.
+    """
+    try:
+        branch = subprocess.check_output(
+            ["git", "rev-parse", "--abbrev-ref", "HEAD"],
+            stderr=subprocess.DEVNULL,
+            text=True,
+        ).strip()
+    except (subprocess.CalledProcessError, FileNotFoundError):
+        return None
+    m = BRANCH_ISSUE_RE.search(branch)
+    return int(m.group(1)) if m else None
+
+
 def parse_args() -> argparse.Namespace:
     p = argparse.ArgumentParser(description="Log a cycle retrospective")
     p.add_argument("--cycle", type=int, required=True)
@@ -230,6 +253,13 @@ def update_summary() -> None:
 def main() -> None:
     args = parse_args()
 
+    # Auto-detect issue from branch name when not explicitly provided
+    if args.issue is None:
+        detected = _detect_issue_from_branch()
+        if detected is not None:
+            args.issue = detected
+            print(f"[retro] Auto-detected issue #{detected} from branch name")
+
     # Reject idle cycles — no issue and no duration means nothing happened
     if not args.issue and args.duration == 0:
         print(f"[retro] Cycle {args.cycle} skipped — idle (no issue, no duration)")