Document workspace concurrency model and per-task lock policy

New doc (docs/architecture/workspace-concurrency.md) explains:
  - why lockMode='none' is the default (cognitive tasks run in
    parallel with no lock)
  - what tasks.requires_filesystem means and how v52 backfills it
  - the coding-role inference safety net during the v52 transition
  - the per-task decision table the resolver implements
  - why company_fallback was removed and what replaces it
  - the type-enforced invariants on the resolution pair
  - failure modes and how heartbeat/task-runner recover

Also adds /docs/architecture/*.md to the .gitignore allowlist so the
docs directory is tracked the same way /docs/*.md and
/docs/proposals/*.md already are.

.gitignore

@@ -55,6 +55,7 @@ oldreference/
 !/README.md
 !/docs/*.md
 !/docs/proposals/*.md
+!/docs/architecture/*.md
 !/CLAUDE.md
 !/AGENTS.md
 !/.planning/**/*.md