docs: prune TODO.md, and correct four claims that were no longer true

TODO.md becomes a TODO again: only work that is actually queued. Five of
its seven sections were finished work, and their reasoning has permanent
homes — the rules moved to AGENTS.md (the baked rates seed a new dataset
only, never a fallback), the rest is in the commit messages and the
specs. 215 lines to 131.

The four corrections, all of them things a reader would have believed:

- README said the install prompt could not work because the manifest
  "only ships an SVG icon". It has shipped 192x192 and 512x512 PNGs since
  f50c65b. Now stated as unverified rather than incomplete, which is what
  it actually is.
- TODO said no browser had seen the two rule-editing surfaces, while its
  own section 5 listed both as seen and confirmed.
- TODO said "merged to main" when the last three commits were on a
  branch.
- scripts/screenshot.ts explained its own existence by saying
  ARCHITECTURE.md and TODO.md claim no browser can run here. Both now say
  the opposite — partly because of that script.

ARCHITECTURE.md's gotchas section now says AGENTS.md holds the
enforceable form of those rules and this holds the mechanism, so a reader
hitting the overlap knows which is canonical.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

scripts/screenshot.ts

@@ -7,11 +7,12 @@
  *                             [--eval=<js>|--eval-file=<path>] [--dark] [--reload]
  *                             [--profile=<dir>] [--eval-after-file=<path>]
  *
- * Why this exists: docs/ARCHITECTURE.md and docs/TODO.md both say nothing here can run a
- * browser, so appearance has never been verified. Chrome IS available, but its
- * plain `--screenshot` flag fires on the load event — before this app's async
- * IndexedDB read resolves, which captures an empty page. Driving CDP directly
- * lets us wait, act, and read console errors.
+ * Why this exists: the docs used to say nothing here could run a browser, so
+ * appearance went unverified for a long time. Chrome IS available — but its
+ * plain `--screenshot` flag fires on the load event, before this app's async
+ * IndexedDB read resolves, so it captures an empty page, which is how that
+ * belief survived. Driving CDP directly lets us wait, act, and read console
+ * errors.
  *
  * This does NOT replace a human: it cannot judge feel, and it renders with one
  * font stack on one platform. It does prove a layout exists and is not broken.