fix(cli): plugin UX — anchor relative specs, reconcile by installed state, guide blocked git builds

- Relative path specs (., ../plugin, file:/link: forms) anchor to the
  invoking directory before forwarding: pnpm's cwd is the profile dir, so a
  bare 'add .' from a plugin checkout used to self-link the profile (exit 0,
  nothing installed). Bare paths stay bare and prefixed specs keep their
  prefix, preserving pnpm's link-vs-copy semantics.
- dsh.plugins reconciles against the INSTALLED state on every successful
  pnpm run, not the dependency diff: an update whose new version gains
  dsh.patch activates the layer; a version that drops it (or a removal)
  deactivates it. Template bundles are never touched.
- A failed pnpm run now names the profile directory, and a git-spec failure
  explains pnpm >=10's prepare-script block with a pointer at the profile's
  pnpm-workspace.yaml allowBuilds (turtle-ui's prepare-based git install is
  the reference consumer); reference README documents all three.

apps/cli/reference/README.i18n.yaml

@@ -2,5 +2,5 @@
 # side as of the last confirmed-consistent state. Both languages carry equal authority;
 # after editing either side, bring the other along and re-record with:
 #   pnpm run verify-translation-pairing --write apps/cli/reference/README.md
-README.md: 583ee093119eb01ff7b37a6aced7b1d9d8cedc92
-README.zh.md: 452dee18ec94e05bcff269a5f24fe0d455c6fe96
+README.md: 25c74bc6020aec409381796e129873bbdc937436
+README.zh.md: fd29f8f6a29d5858e6c9b6b66e21d5b92733480c