fix(ultraplan-local): CRITICAL — worktree isolation for parallel sessions

Phase 2.6 previously launched parallel claude -p sessions in the same
working directory, causing git race conditions and repository corruption.

Changes:
- Add Phase 2.55 (pre-flight safety checks): clean tree, plan file
  tracking, scope fence overlap validation, stale worktree cleanup
- Rewrite Phase 2.6 with git worktree isolation: each parallel session
  gets its own worktree and branch, merged back sequentially
- Add merge conflict detection and abort (no silent data loss)
- Add unconditional worktree cleanup (even on failure)
- Add hard rules 11-13 (worktree mandatory, cleanup, sequential merge)
- Session-scoped progress file naming for --session mode
- Update headless launch template with worktree support and cleanup trap
- Bump version to 1.5.0

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

plugins/ultraplan-local/templates/headless-launch-template.md

@@ -17,23 +17,55 @@ set -euo pipefail
 # Prevent accidental API billing — remove this line if you intend to use API credits
 unset ANTHROPIC_API_KEY
 
+REPO_ROOT="$(git rev-parse --show-toplevel)"
 PLAN_DIR="{session_dir}"
 LOG_DIR="{session_dir}/logs"
-mkdir -p "$LOG_DIR"
+WORKTREE_BASE="{session_dir}/worktrees"
+mkdir -p "$LOG_DIR" "$WORKTREE_BASE"
 
-echo "=== Ultraplan Headless Execution ==="
+# Cleanup trap — always remove worktrees on exit (success or failure)
+cleanup_worktrees() {
+  echo ""
+  echo "=== Cleaning up worktrees ==="
+  cd "$REPO_ROOT"
+  for wt in "$WORKTREE_BASE"/session-*; do
+    [ -d "$wt" ] && git worktree remove "$wt" --force 2>/dev/null && echo "Removed: $wt"
+  done
+  git worktree prune
+  git branch --list "ultraplan/{slug}/*" | while read b; do
+    git branch -D "$b" 2>/dev/null
+  done
+  rmdir "$WORKTREE_BASE" 2>/dev/null
+  echo "Cleanup complete."
+}
+trap cleanup_worktrees EXIT
+
+# Pre-flight: verify clean working tree
+if [ -n "$(git status --porcelain)" ]; then
+  echo "ERROR: Working tree is not clean. Commit or stash changes before parallel execution."
+  git status --short
+  exit 1
+fi
+
+echo "=== Ultraplan Headless Execution (Worktree-Isolated) ==="
 echo "Plan: {plan_path}"
 echo "Sessions: {total_sessions}"
+echo "Repo root: $REPO_ROOT"
 echo ""
 
 # --- Wave {N}: Parallel sessions (no dependencies) ---
 echo "--- Wave {N}: {description} ---"
 
-{# For each parallel session in this wave: }
-claude -p "$(cat "$PLAN_DIR/session-{n}-{slug}.md")" \
+{# For each parallel session in this wave, create worktree: }
+git worktree add -b "ultraplan/{slug}/session-{n}" "$WORKTREE_BASE/session-{n}" HEAD
+echo "Worktree created: session-{n} (branch: ultraplan/{slug}/session-{n})"
+
+{# Launch session in its worktree: }
+cd "$WORKTREE_BASE/session-{n}" && claude -p "$(cat "$PLAN_DIR/session-{n}-{slug}.md")" \
   --dangerously-skip-permissions \
   > "$LOG_DIR/session-{n}.log" 2>&1 &
 PID_{n}=$!
+cd "$REPO_ROOT"
 echo "Started session {n}: {title} (PID $PID_{n})"
 
 {# After all parallel sessions in this wave: }
@@ -42,6 +74,25 @@ wait $PID_{n1} $PID_{n2}
 echo "Wave {N} complete."
 echo ""
 
+# --- Merge wave results (sequential) ---
+echo "--- Merging Wave {N} ---"
+cd "$REPO_ROOT"
+{# For each session in the wave, merge its branch: }
+git merge --no-ff "ultraplan/{slug}/session-{n}" \
+  -m "merge: ultraplan session {n} — {title}"
+if [ $? -ne 0 ]; then
+  echo "MERGE CONFLICT: session {n}. Conflicting files:"
+  git diff --name-only --diff-filter=U
+  git merge --abort
+  echo "Aborting. Earlier sessions in this wave are already merged."
+  exit 1
+fi
+git worktree remove "$WORKTREE_BASE/session-{n}" --force
+git branch -d "ultraplan/{slug}/session-{n}"
+echo "Merged and cleaned: session {n}"
+
+git worktree prune
+
 # --- Verify wave results ---
 echo "--- Verifying Wave {N} ---"
 {# For each session in the wave, run its exit condition commands }
@@ -78,3 +129,16 @@ When generating a launch script from this template:
 10. **Billing preamble.** Prepend `unset ANTHROPIC_API_KEY` with a comment at
     the top of the script to prevent accidental API billing. Users who intend
     to use API credits can remove this line.
+11. **Worktree isolation is mandatory.** Every parallel wave MUST use git
+    worktrees. Each session gets its own worktree and branch. Never launch
+    parallel `claude -p` sessions in the same working directory.
+12. **Cleanup trap on EXIT.** The generated script MUST include a `trap` on
+    EXIT that removes all worktrees (`git worktree remove --force`) and prunes
+    branches, even if the script fails or is interrupted.
+13. **Sequential merge after each wave.** After all sessions in a wave complete,
+    merge their branches back to the main branch one at a time. Abort on merge
+    conflict — do not force-resolve.
+14. **Clean working tree before worktrees.** Add a `git status --porcelain`
+    check at the top of the script. Fail if the working tree is dirty.
+15. **Absolute paths for logs.** Log file paths must be absolute (resolved from
+    `$REPO_ROOT`), not relative to any worktree.