Extend heartbeat worktree-reconciliation with cross-agent branch advance detection

From 2026-06-16 self-reflection. Atomic HEARTBEAT.md edit. Workspace-internal (no PR). ADD a new doctrine subsection to HEARTBEAT.md under the worktree-reconciliation step (Step 1 area, or wherever current worktree-reconciliation lives). Title: 'Cross-agent branch advance detection.' CONTENT: During the worktree-reconciliation heartbeat step, for each non-main worktree, run: cd <worktree> git fetch origin <branch> --quiet 2>/dev/null behind=$(git rev-list --count HEAD..origin/<branch>) # alternative: git status --porcelain=v2 -b | awk '/^# branch.ab/ {print $3}' If behind > 0, log a worktree-drift event with: worktree path, local HEAD short SHA, upstream HEAD short SHA, behind-count, last upstream commit author (git log -1 origin/<branch> --format='%an <%ae>'). Do NOT automatically fast-forward — cross-agent pushes may have force-push or rebase semantics that need human review. CASE STUDY (2026-06-15T16:25:36Z): texture-coding-agent pushed c201fb07 to winston/bolt-1193-brush-zoom-auto-densify (PR #11790, Richie V-notch repro fix). Cato's mono-bolt-1193 worktree was at 0f8a76d. The 04:24Z next-morning heartbeat reported worktree-reconciliation as clean — local HEAD unchanged was misread as in-sync. The PR turned CONFLICTING at 18:45Z. The reconciliation step had no git-fetch + behind-count so the drift was invisible until the next reflection pulled gh pr view. Cost: ~12h blind window. A git-fetch + behind-count per worktree costs ~1-3s and would have surfaced the drift in real time. ATOMIC STEPS: 1. Read current HEARTBEAT.md to locate the worktree-reconciliation section. 2. Insert the new subsection. 3. Verify edit clean. 4. PATCH this task to completed via bin/fleet-task-patch.sh with file:line of the new subsection and total line count.