docs(agents): slim AGENTS.md to repo-global rules, single-source the API reference

AGENTS.md (546 -> 63 lines) now carries only project-global development
rules: hard constraints, mount-smoke CI, DSH 0.1.2-alpha.1 adaptation
notes, npm release, a quick-rule list, and a docs/tests map. The
consumer plugin API reference lives solely in
docs/external-plugin-guide.md.

Reference sync:
- README/README_EN: collapse the dual 'full integration docs' links to
  the guide; AGENTS.md stays linked as the repo rules
- shell-presets.ts / mount.e2e.ts comments: skinning contract pointer
  moved from AGENTS.md §8 to guide §12

src/client/shell-presets.ts

@@ -5,7 +5,7 @@
  * the SAME generic strip variable and CSS-injection mechanism — adding a
  * shell is adding data, never a code path.
  *
- * Inclusion rule (maintained in AGENTS.md §8): only shells that (a) appear
+ * Inclusion rule (maintained in docs/external-plugin-guide.md §12): only shells that (a) appear
  * in this repo's issues/PRs (a user actually hit a problem) and (b) have
  * 100+ GitHub stars (a real user base). The mechanism is opt-in: auto
  * detection never applies a preset — the settings badge only SUGGESTS it.