docs: add operating manual, fix cross-references after doc restructure

- Commit CLAUDE.md + AGENTS.md so a fresh clone has the operating manual and
  the ABI/no-game-logic rules (trimmed of stale phase status; the personal
  pd_port path removed; points at the split docs).
- Sweep every stale doc reference across the tree after the restructure:
  PCPortResearch.md -> internals.md / dev/findings.md,
  PORT-LEARNINGS.md -> porting-notes.md,
  AGENT-WORKFLOW.md -> dev-process.md,
  docs/{GRAPHICS-BACKLOG,HANDOFF-ARCHIVE,GE-ENV-PROBES,LEVEL-*,TEXTURE-GLITCH-ANALYSIS,AUDIT-M6-*,AUDIO-PLAN}.md -> docs/dev/.
- Drop the superseded docs/PLAN-linear-level-sweep.md.
- docs/HANDOFF.md + docs/BACKLOG.md stay local-only (rolling working files);
  CLAUDE.md now handles their absence in a fresh clone.

docs/dev/HANDOFF-ARCHIVE.md

@@ -81,7 +81,7 @@ storage-room void is NOT closed doors failing to render.
 
 **Priority (user, M-11): get BUNKER1 — then every level — playable
 start-to-finish with no crashes. Fix crashes/hangs first; cosmetic
-rendering (D74/D75/D76/D114/D116) is logged in `docs/GRAPHICS-BACKLOG.md`
+rendering (D74/D75/D76/D114/D116) is logged in `docs/dev/GRAPHICS-BACKLOG.md`
 and comes later.** D120 (blood-stain converter) is the nearest real
 converter gap; extending `d43_emit.py` for opcode-0x18 is a good
 subagent brief. After BUNKER1: walk the objective/exit path to a level
@@ -107,13 +107,13 @@ transition, then the next stage.
    boot may spawn Bond holstered — may need forced weapon state or the
    attract path to get the gun on screen. Gate to playability.
 3. **The HUD/text X-mirror (D116)** — DEPRIORITISED, parked in
-   `docs/GRAPHICS-BACKLOG.md`. M-11 confirmed the ammo digits use the
+   `docs/dev/GRAPHICS-BACKLOG.md`. M-11 confirmed the ammo digits use the
    SAME `textrelated.c` path (no separate renderer — the M-8 premise was
    wrong) and re-hit the same contradiction: every stage verified
    non-mirrored, glyphs still flip. **Do not re-static-trace.** Next
    attempt needs a RenderDoc/apitrace capture or the asymmetric-1-texel
    experiment — nothing else. All cosmetic rendering issues (D74 dead
-   wrap-block, D75/D76/D77) now tracked in `docs/GRAPHICS-BACKLOG.md`;
+   wrap-block, D75/D76/D77) now tracked in `docs/dev/GRAPHICS-BACKLOG.md`;
    they rank BELOW level-progression and crash work.
 4. **Rest of the `struct player` offset pass** — D115 fixed the HIGH
    `gunfire.c` THROW* bugs; #6 (watch-preview Model pool) + the broader