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>

.wiki/index.md

@@ -13,6 +13,7 @@ Catalog of all wiki pages. One line per page, organized by type. Updated on ever
 ## Concepts
 
 - [active-platform-decision.md](concepts/active-platform-decision.md) — why `active-platform` is a skill (not a memory entry); why default = Windows; how it's wired into `project-bootstrap`
+- [bootstrap-claude-md-merge.md](concepts/bootstrap-claude-md-merge.md) — project-bootstrap@1.3.0 — Step 5 upgrade path becomes idempotent merge (read → diff vs template → confirm → append missing); fixes silent gap where pre-1.2.0 projects never picked up new canonical triggers (`check across all projects`, `we're on Windows`)
 - [bootstrap-manifest.md](concepts/bootstrap-manifest.md) — record of which `project-bootstrap` / `setup-wiki` / `setup-tasks` versions initialized this project's `.wiki/` and `.tasks/` layout (overwritten on re-bootstrap; history in git)
 - [build-notes.md](concepts/build-notes.md) — why `build.ps1` exists alongside `build.sh`; PS 5.1 backslash-in-zip gotcha; how to extract a `.skill`
 - [install-portability.md](concepts/install-portability.md) — `install.sh` / `build.sh` rewritten to drop `mapfile` (bash 4+) and `find -printf` (GNU only) so stock macOS (bash 3.2 + BSD find) works