Make the repo legible to someone seeing it for the first time

The README claimed screenshots were "coming" and stopped describing the
project around phase 10, so the world-state engine, the retry/undo state
revert and the egress work — the most interesting parts — were invisible.

- Screenshots of the play screen, Insights, the schema editor and the
  script editor, captured from the running app.
- Document the world-state engine, undo/retry rewind, and the two measured
  performance fixes; correct the context assembly order to match
  context/builder.py; drop the stale "phases 1-6 complete" note.
- Add CI (backend pytest, frontend lint + build, Docker image build) and
  its badge. Nothing ran the 151 tests but me.
- Move CODE_REVIEW_FINDINGS.md to docs/self-review.md and say up front
  that every correctness finding is resolved.
- Drop a dead import that the newly-wired lint flagged.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_015sygnUWH8WnaoKPDp7hXM1

frontend/src/pages/Scripts.jsx

@@ -1,7 +1,7 @@
 import { useEffect, useState } from 'react'
 import { useNavigate } from 'react-router-dom'
 import { api } from '../api'
-import { downloadJSON, pickJSONFile, useToast } from '../components'
+import { pickJSONFile, useToast } from '../components'
 
 export default function Scripts() {
   const [scripts, setScripts] = useState(null)