feat(project-bootstrap): v1.3.0 — idempotent CLAUDE.md merge on upgrade

Step 5 was binary on upgrade (append whole template / leave alone), so
projects bootstrapped before v1.2.0 silently missed new canonical triggers
(`check across all projects`, `we're on Windows`) on re-run. Now upgrade
reads existing CLAUDE.md, substring-diffs vs template, preserves a
deliberately-pinned platform line, and appends only missing lines after
explicit confirm. Re-runs are no-ops.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

skills/project-bootstrap/README.md

@@ -44,7 +44,7 @@ projects bootstrapped at different times debuggable.
 | `README.md` | minimal stub | Skipped if file exists. |
 | `.wiki/` | delegated to `setup-wiki` | Karpathy LLM Wiki layout — `CLAUDE.md`, `index.md`, `log.md`, `overview.md`, `raw/`, `entities/`, `concepts/`, `packages/`, `sources/`. |
 | `.tasks/` | delegated to `setup-tasks` | Canonical board — `STATUS.md` plus per-task `<task-slug>.md` files. |
-| `CLAUDE.md` | `assets/CLAUDE.md.template` | Skill triggers (`use superpowers`, `use project wiki`, etc.). On non-Windows hosts, swap the `we're on Windows` line for `we're on Linux` / `we're on macOS`. |
+| `CLAUDE.md` | `assets/CLAUDE.md.template` | Skill triggers (`use superpowers`, `use project wiki`, etc.). On non-Windows hosts, swap the `we're on Windows` line for `we're on Linux` / `we're on macOS`. On upgrade, the template is treated as a canonical set and merged idempotently — only missing trigger lines are appended after user confirm. Re-runs are no-ops. |
 | `.wiki/concepts/bootstrap-manifest.md` | generated | Records which skill versions initialized the project, so cross-project layout drift is debuggable. |
 
 ## Workflow