docs(ultraplan-local): update README for v1.5.0 worktree isolation

- Version badge 1.4.0 → 1.5.0
- Rewrite parallel execution section to document worktree isolation,
  pre-flight checks, sequential merge, and automatic cleanup
- Update plugin.json version reference in directory tree

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

plugins/ultraplan-local/README.md

@@ -1,6 +1,6 @@
 # ultraplan-local and ultraexecute-local — Plan Deep, Execute Clean
 
-![Version](https://img.shields.io/badge/version-1.4.0-blue)
+![Version](https://img.shields.io/badge/version-1.5.0-blue)
 ![License](https://img.shields.io/badge/license-MIT-green)
 ![Platform](https://img.shields.io/badge/platform-Claude%20Code-purple)
 
@@ -143,23 +143,27 @@ Reads a plan from `/ultraplan-local` and implements it with strict discipline. N
 | **Foreground** | `/ultraexecute-local plan.md --fg` | Force sequential, ignore Execution Strategy |
 | **Single session** | `/ultraexecute-local plan.md --session 2` | Execute only session 2 from Execution Strategy |
 
-### Session-aware parallel execution
+### Session-aware parallel execution (worktree-isolated)
 
 When a plan has an `## Execution Strategy` section (auto-generated by `/ultraplan-local` for plans with > 5 steps), `/ultraexecute-local` automatically:
 
-1. Parses sessions, waves, and scope fences from the plan
+1. **Pre-flight checks** — validates clean working tree, plan file tracked in git, no scope fence overlaps between parallel sessions, no stale worktrees
-2. Launches parallel `claude -p "/ultraexecute-local --session N plan.md"` per session per wave
+2. **Creates git worktrees** — each parallel session gets its own isolated worktree and branch (`ultraplan/{slug}/session-{N}`)
-3. Waits for each wave to complete before starting the next
+3. Launches `claude -p` per session per wave, each in its own worktree
-4. Aggregates results and runs master verification
+4. **Merges branches back** sequentially with `--no-ff` after each wave completes
+5. **Cleans up** worktrees and branches unconditionally (even on failure)
+6. Runs master verification on the merged result
 
 ```
-Wave 1: Session 1 (Foundation) + Session 2 (Middleware)  -- parallel
+Wave 1: Session 1 (worktree-1) + Session 2 (worktree-2)  -- parallel
-         ↓ both complete
+         ↓ both complete → sequential merge to main
-Wave 2: Session 3 (Integration)                          -- sequential
+Wave 2: Session 3 (worktree-3)                             -- sequential
-         ↓ complete
+         ↓ complete → merge to main
-Master verification
+Cleanup worktrees + Master verification
 ```
 
+Each session operates in complete filesystem isolation — no shared git index, no race conditions, no data loss. If a merge produces conflicts, the merge is aborted and conflicting files are reported.
+
 Use `--fg` to force sequential execution even when a plan has an Execution Strategy.
 
 ### Billing safety
@@ -310,7 +314,7 @@ The plugin minimizes Opus usage by front-loading cheap Sonnet exploration.
 ```
 ultraplan-local/
 ├── .claude-plugin/
-│   └── plugin.json              # Plugin manifest (v1.4.0)
+│   └── plugin.json              # Plugin manifest (v1.5.0)
 ├── agents/                      # 13 specialized agents
 │   ├── architecture-mapper.md   # Codebase structure and patterns
 │   ├── dependency-tracer.md     # Import chains and data flow