docs(readme): bring the README up to 6.4.0

- Changelog list: v6.0.0 to v6.4.0 replace v5.5.0 to v5.8.0.
- Profile system: custom profiles live in voyage-profiles/ (repo root or
  ~/.claude), never lib/profiles/ (resolver.mjs findProfilePath); env is
  not read on a resume; brief phase_signals come before the profile.
  docs/profiles.md had the same wrong path.
- Annotation: the install-section paragraph described the v5.0.2 UX and
  linked a dead anchor; the v5.0.3 history note moved to a CHANGELOG pointer.
- Architecture slot replaces "Disable the architect bridge"; the producer
  is no longer distributed and any Handover 3 producer fits.
- Stale facts fixed: review severities, seven handovers, session titles for
  six commands, the brief's framing field, Upgrading since v6.0.0, the
  npm test path, the tree root and docs list, the add-an-agent steps.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

docs/profiles.md

@@ -131,7 +131,10 @@ unexpected env-var inheritance in CI.
 
 ## Custom profiles
 
-Drop a YAML file at `lib/profiles/<name>.yaml` to define a new tier.
+Drop a YAML file at `voyage-profiles/<name>.yaml` in the repo root, or at
+`~/.claude/voyage-profiles/<name>.yaml`, to define a new tier (the repo root
+is tried first). `lib/profiles/` holds only the four built-ins; a custom file
+placed there is never found.
 The validator (`lib/validators/profile-validator.mjs`) enforces:
 
 - Every `phase_models[].phase` must be a known phase enum: