Reorganize docs: simple README + organized docs/

Removed docs/stage-1-auth-api.md -- an early planning doc, badly out
of date (predates passwordless auth, statuses, the inbox, and
everything after).

README.md is now just: what this is, a pointer to docs/, an end-user
getting-started guide (Docker up, first-time login via the bundled
Mailpit catcher, adding a passkey), and provenance -- everything else
it used to carry moved out:

- docs/api.md -- the full REST API reference (auth, passkeys,
  projects, cards, statuses, error shape) + the curl walkthrough.
- docs/setup.md -- running without Docker, every environment
  variable, the test suite.
- docs/architecture.md -- backend file layout; points to
  web/README.md for the frontend, which already documented itself in
  enough depth to stand alone.
- docs/history.md -- the stage-by-stage feature log, with a new row
  for this session's refactoring work (which hadn't been logged yet).
- docs/README.md -- an index tying the above together.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

docs/README.md

@@ -0,0 +1,15 @@
+# Documentation
+
+Technical documentation for the PHP Project Manager. Start with the root
+[README](../README.md) if you just want to run the app.
+
+- **[API reference](api.md)** — every REST endpoint: auth (magic links,
+  passkeys), projects, cards, statuses, error shapes, and a curl walkthrough.
+- **[Architecture](architecture.md)** — backend layout and how the pieces fit
+  together. Frontend architecture lives in
+  [web/README.md](../web/README.md) instead, since it's a large enough topic
+  on its own.
+- **[Setup & configuration](setup.md)** — running without Docker, every
+  environment variable, and the test suite.
+- **[Development history](history.md)** — a stage-by-stage log of how the app
+  got here.