docs(engine): simplify execution tiers — Monty-only for CodeAct/RLM

Restructure phases 6-8 to clarify execution model:

- Monty is the sole Python executor for CodeAct/RLM. No WASM or Docker
  Python runtimes for LLM-generated code.
- WASM sandbox is for third-party tool isolation (existing infra, Phase 8)
- Docker containers are for thread-level isolation of high-risk work (Phase 8)
- Two-phase commit moves to Phase 6 (integration) at the adapter boundary

Phase renumbering:
- Old Phase 6 (Tier 2-3) → removed as separate phase
- Old Phase 7 (integration) → Phase 6
- Old Phase 8 (cleanup) → Phase 7
- New Phase 8: WASM tools + Docker thread isolation (infra integration)

Updated progress table: Phases 1-5 marked DONE with test counts and commits.

Co-Authored-By: Claude Opus 4.6 (1M context) <[email protected]>

crates/ironclaw_engine/src/types/step.rs

@@ -39,15 +39,16 @@ pub enum StepStatus {
 }
 
 /// Which execution tier handles the step's code/actions.
+///
+/// Monty is the sole CodeAct/RLM executor. WASM and Docker are used for
+/// third-party tool isolation and thread sandboxing (Phase 8), not for
+/// running LLM-generated Python.
 #[derive(Debug, Clone, Copy, PartialEq, Eq, Serialize, Deserialize)]
 pub enum ExecutionTier {
-    /// Tier 0: structured tool calls.
+    /// Structured tool calls (JSON action calls from LLM).
     Structured,
-    /// Tier 1: embedded Python via Monty.
+    /// Embedded Python via Monty (CodeAct/RLM pattern).
     Scripting,
-    // Future tiers:
-    // Wasm,       // Tier 2: WASM sandbox
-    // Container,  // Tier 3: Docker container
 }
 
 /// A single execution step within a thread.