Make distributable: config.toml support, blanked Matrix defaults, env creds fallback

- ~/.config/coe-mirror/config.toml (tomllib, Python 3.11+ required)
- Precedence uniform for every key: env var > config file > hardcoded default
- API_BASE, OWNERS, PCLOUD_TARGET, PENDING_DIR, matrix homeserver/room resolved
  via _resolve() helper
- Matrix homeserver/room now default to empty; _send_matrix short-circuits when
  either is blank so forks don't spam the original author's room
- _fetch_creds falls back to COE_MIRROR_MATRIX_USER/PASSWORD env vars when the
  Infisical creds binary is unavailable (required for external distribution)
- CLI subcommands: config init (writes example TOML, won't overwrite), config path
  Bare invocation and 'run' still work unchanged for the systemd timer
- README rewritten as a public quick-start with the precedence table and Matrix
  as opt-in
- Tests: 3 new for config precedence, 1 for Matrix silent no-op when blank,
  2 for creds env fallback (26 total, all pass)

DevTrack #1285

.gitignore

@@ -5,3 +5,5 @@ __pycache__/
 .pytest_cache/
 *.egg-info/
 .active-item.json
+.workflow-state.json
+agent-spawns/