docs: correct solo-mission count 21 -> 20 (level_54 'Cuba' is the ending-credits sequence, not a mission)

The campaign has 20 playable missions (SP_LEVEL_MAX // 20 in
bondconstants.h; mission_folder_setup_entries in front.c has exactly 20
entries). The old '21' counted -level_54 (LEVELID_CUBA), which the front
end jumps to after the final mission for the extended cast display +
credits scroll (do_extended_cast_display / bondviewRenderCredits, D129) -
it is not a selectable mission.

- README, docs/index.md, .github/release-notes.md,
  docs/security-and-fidelity-status.md: 21 -> 20 in current-status claims
- AGENTS.md phase status: same correction
- docs/dev/LEVEL-STATUS.md + LEVEL-PLAYTEST.md: relabel the level list;
  counting note explains historical '21/21 PASS' sweep results = 21 stage
  boots (20 missions + ending sequence)
- social-preview card: stat line 21 -> 20, PNG regenerated (1280x640)

AGENTS.md

@@ -51,8 +51,9 @@ Run `/linkcheck` for this sweep. Record new findings in `docs/dev/findings.md`
 
 - **Phase 0–1.5:** done. Build system, boot chain, OS-shim layer, fast3d
   integration, first frames, full intro rendering.
-- **Phase 2 (rendering):** in progress. All 21 solo levels load + render +
-  survive an unattended window; front end (menu → mission select → briefing →
+- **Phase 2 (rendering):** in progress. All 20 solo missions (plus the
+  ending-credits sequence, `-level_54`) load + render + survive an
+  unattended window; front end (menu → mission select → briefing →
   start) is functional; file-backed EEPROM saves work. Cosmetic defects are
   parked in `docs/dev/GRAPHICS-BACKLOG.md`.
 - **Phase 3 (audio + input):** input layer done (`port/src/input.c`); polish