docs(architecture): add diagrams and implementation playbook

Added three new documents with nine Mermaid diagrams to make the strangler
fig strategy visible:

- README: container topology diagram at the start, with the proxy entry point
  and three seams labelled
- docs/architecture.md: five diagrams tracing the exact implementation:
  - The four seams and who holds authority at each boundary
  - How by-id read goes through the resolver, but list-read bypasses it
  - Case lifecycle state machine (the strategy in one picture)
  - Take-ownership sequence with failure windows annotated
  - Write-through error round-trip showing zero validation logic crossed
- docs/playbook.md: how to apply this to a production system:
  - Write-path decision tree (five read/write patterns)
  - Cutover ordering diagram (side-effects-free first, least recoverable last)
  - Seven transferable rules with pointers to the files that demonstrate them
  - Scope diagram of what's proven vs. left as your decisions

Resolved all 13 dangling § citations (to an absent spec doc) by linking to
the actual files or dropping them. Replaced portal-frontend/README.md
boilerplate with accurate content. All diagrams parse and link-check clean.

Co-Authored-By: Claude Haiku 4.5 <noreply@anthropic.com>

docs/sync-not-implemented.md

@@ -21,6 +21,7 @@ consequences, both intentional:
    honest substitute for a sync that does not exist.
 2. **Ownership release is blocked once edits exist.** `DELETE
    /api/worklist/owned/{id}/ownership` returns `409` once `domain_writes_since
-   > 0` (§7.6) — releasing would silently discard those edits, since there is
+   > 0` (`ReleaseOwnershipHandler`) — releasing would silently discard those
+   edits, since there is
    no sync to have propagated them back to legacy first. The `409` is the cost
    of the missing sync made visible, rather than a data-loss bug made invisible.