Fix doc cross-links for the hierarchy; add package-path + shape gates

Merge brought in the RFC-classification reorg and two new doc gates;
rewrite every drifted packages/<name> cross-link (Markdown link targets,
moved-README relative depths, and .ts comment paths) to the grouped paths.

Add two doc-sync/hygiene gates so the manual checks this restructure
needed become automated:
- verify-package-paths.ts: flags a packages/<path> reference (in Markdown
  or a .ts comment/string) that does not resolve AND names a real package
  in a segment — i.e. a stale path to a MOVED package. A path naming a
  non-existent package (a forward-looking proposal) is left alone, so it
  applies uniformly across proposed/implemented/rejected.
- check-workspace-constraints: assert the packages/<group>/<pkg> depth-2
  shape (group dirs carry no package.json; no flat or over-nested
  packages). Group names stay open; only the shape is fixed.

package.json

@@ -27,6 +27,7 @@
     "verify-md-wrap": "tsx scripts/verify-md-wrap.ts",
     "verify-md-links": "tsx scripts/verify-md-links.ts",
     "verify-doc-refs": "tsx scripts/verify-doc-refs.ts",
+    "verify-package-paths": "tsx scripts/verify-package-paths.ts",
     "verify-rfc-classification": "tsx scripts/verify-rfc-classification.ts",
     "verify-type-equiv": "tsx scripts/verify-type-equiv.ts",
     "gen-cordis-catalog": "tsx scripts/gen-cordis-catalog.ts",
@@ -34,7 +35,7 @@
     "gen-module-graph": "tsx scripts/gen-module-graph.ts",
     "verify-module-graph": "tsx scripts/gen-module-graph.ts --check",
     "constraints": "tsx scripts/check-workspace-constraints.ts",
-    "doc-sync": "pnpm run doc-typecheck && pnpm run verify-cordis-catalog && pnpm run verify-md-wrap && pnpm run verify-md-links && pnpm run verify-doc-refs && pnpm run verify-rfc-classification && pnpm run verify-type-equiv",
+    "doc-sync": "pnpm run doc-typecheck && pnpm run verify-cordis-catalog && pnpm run verify-md-wrap && pnpm run verify-md-links && pnpm run verify-doc-refs && pnpm run verify-package-paths && pnpm run verify-rfc-classification && pnpm run verify-type-equiv",
     "hygiene": "pnpm run knip && pnpm run publint && pnpm run constraints",
     "demo:echo": "node --expose-internals --import tsx examples/echo-agent/start.ts",
     "demo:coding": "node --expose-internals --import tsx examples/coding-agent/start.ts",