T5.5: Update docs to reflect Phase 5 package structure

Rewrote ARCHITECTURE.md with a new package-map section and updated
Mermaid diagram showing src/app Service, src/scheduler, src/runner,
src/storage, src/platform/autostart, and src/ui replacing the old
src/core + src/gui split. Main-flows section documents the event-driven
model and ErrorOccurred surfacing.

Rewrote TESTS.md to cover all nine current test files (domain, app ×4,
storage, scheduler, runner ×2, platform/autostart ×2, ui); every test
function is documented. Removed stale src/core references.

Updated README Project Layout from the two-package summary to the full
per-package description.

Marked T5.5 complete in REFACTORING.md.

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

docs/REFACTORING.md

@@ -281,7 +281,7 @@ Track progress here. Mark tasks complete as they land and pass review.
 - [x] T5.2 — Introduce `autostart.Manager` interface
 - [x] T5.3 — Fill test gaps (folder filtering, cleanup, migration, concurrency)
 - [x] T5.4 — Run `go test -race ./...` clean on both platforms
-- [ ] T5.5 — Update docs (ARCHITECTURE.md, TESTS.md, README)
+- [x] T5.5 — Update docs (ARCHITECTURE.md, TESTS.md, README)
 
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