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.

docs/rfc/rejected/simplification/2026-06-20-drop-acp-session-load.md

@@ -18,7 +18,7 @@ For now, ACP starts fresh sessions only. `initialize` advertises `loadSession: f
 - `initialize` does not advertise load support.
 - The `session/load` handler, loading-id tracking, cwd preflight for loaded sessions, and load replay tests are removed.
 - Snapshot fixtures no longer rely on load replay presentation.
-- [ACP docs](../../../../packages/acp/README.md) describe fresh-session support only.
+- [ACP docs](../../../../packages/ui/acp/README.md) describe fresh-session support only.
 
 ## What we give up