docs: promote overview.md to root ARCHITECTURE.md + add update discipline table

Move docs/architecture/overview.md to ARCHITECTURE.md at the repo root —
it is the primary cross-codebase architecture doc (four-codebase diagram,
inter-codebase contracts, secrets map, build matrix, test strategy, where-to-look
table) and belongs at the root alongside STATUS.md, ROADMAP.md, etc.

Update relative paths inside the file (../../crates/ → crates/, etc.).
Update CHANGELOG.md's one active reference to the old path.

Add a "Living docs — update discipline" table to CLAUDE.md that maps every
ALLCAPS.md file to the area it covers and the trigger for updating it. This
closes the loop on the ALLCAPS.md documentation system.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

CHANGELOG.md

@@ -135,7 +135,7 @@ two confirmed bugs).
   the `.form-grid` cards above. Removes the visual rhythm break at the
   2-col → full-width transition. The popup surface is unchanged.
 - **Documentation refreshed for v0.5.0 (doc audit, 14 findings).**
-  `docs/architecture/overview.md` now describes four codebases (the
+  `ARCHITECTURE.md` now describes four codebases (the
   `relicario-server` pre-receive hook crate is no longer invisible);
   `CLAUDE.md` project tree and roadmap reflect current state;
   `docs/SECURITY.md` names the server crate and its `verify-commit` /