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-assembled-assistant-messages-only.md

@@ -17,7 +17,7 @@ ACP `session/load` can replay prior assistant messages as complete content block
 ## Acceptance criteria
 
 - `SessionEventMap` drops `assistant/chunk`, or marks it as non-persisted if a transitional live event is needed.
-- [Session persistence docs](../../../../packages/session-persistence/README.md) no longer require every stream chunk to be stored verbatim.
+- [Session persistence docs](../../../../packages/session-persistence/session-persistence/README.md) no longer require every stream chunk to be stored verbatim.
 - `llm-replay` and ACP snapshots use an explicit replay fixture format or sidecar for model chunks.
 - `session/load` renders completed assistant messages from `assistant/message`.
 - Stored logs get much smaller and remain `seq`-contiguous without chunk holes.