docs(rlm): land Hetun design + helper layer + Sakana research methodology

Captures the full RLM-fundamental story across the design doc, MODES.md,
and the Hetun prompt. Tracking issues are now #46–#55 (helper layer
filed as #53, Hetun as #54, vendoring as #55).

What this nails down:

- **Hetun mode** is added at the END of the Tab cycle (Plan → Agent →
  YOLO → Hetun → Plan), not as a Plan replacement. Default landing mode
  is unchanged so people don't accidentally start there. Plan stays as
  it is.

- **Mission-level approval, not block-level.** Hetun runs a research
  phase, presents one mission card, and only executes after explicit
  user approval. Inside the execution turn the repl block runs straight
  through with no per-block prompts — that's the whole point of the
  mode.

- **The user's configured model is left alone on enter/exit.** Pro/max
  users stay on Pro/max. The flash-as-coordinator behaviour is internal
  to the runtime (ZIGRLM_RLM_CMD always points to flash regardless of
  mode). No global model swap.

- **No /hetun slash command.** Tab cycles into the mode; /plan keeps
  switching to Plan as today.

- **The helper layer (#53) is fundamental, not aleph-derived.** A
  curated ~20-function ctx-helper module + AST-validated Python sandbox
  baked into the repl runtime so a single block can load → slice → fan
  out flash queries → aggregate without crossing tool boundaries.
  Inspired by aleph's pattern but our own native primitive — not a port.

- **Hetun research methodology adopts Sakana's Fugu patterns.** The
  research phase is recursive novelty sampling + hierarchical narrative
  tree synthesis + multi-detector cross-verification (flash for
  breadth, Pro for depth) + hypothesis-verification loop. Not "fan out
  8 fixed queries". This is what makes "Plan + Recursive Agents"
  meaningful versus a flash-coordinator wrapper.

- **No version-number framing anywhere.** The plan ships as one cohesive
  RLM landing across #46/#48/#49/#50/#53/#54/#55 — order is dependency,
  not release schedule. We keep shipping.

- **Auto-compaction stays automatic.** Removed a manual /compact nag
  from the Hetun prompt; the existing coherence + capacity system
  already handles this.

Files:
  docs/rlm-design.md          new — full design doc with Hetun details
  docs/research-react-vs-rlm.md  new — supporting research treatment
  docs/MODES.md               4-mode cycle, Hetun added at end, Plan kept
  crates/tui/src/prompts/hetun.txt  prompt teaching the recursive-novelty
                              + hierarchical-synthesis + verification-loop
                              rhythm, mission-card structure, two-step gate
  .gitignore                  ignore .claude/scheduled_tasks.lock runtime

Closes nothing yet — implementation lands across the tracking issues.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

.gitignore

@@ -64,3 +64,6 @@ project_overhaul_prompt.md
 
 # Companion app (tracked separately)
 apps/
+
+# Claude Code runtime artifacts
+.claude/scheduled_tasks.lock