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/.claude-plugin/plugin.json

@@ -1,7 +1,7 @@
 {
   "name": "ultraplan-local",
   "description": "Deep implementation planning with interview, specialized agent swarms, external research, adversarial review, session decomposition, and headless execution support.",
-  "version": "1.4.0",
+  "version": "1.5.0",
   "author": {
     "name": "Kjell Tore Guttormsen"
   },