chore(engine): remove legacy Playbook doc type, superseded by Skill

Drop DocType::Playbook variant and all references — playbook extraction
mission was already renamed to skill extraction in the previous session.
Updates CLAUDE.md, architecture docs, context builder, retrieval weights,
mission comments, and store adapter path mapping.

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

docs/engine-v2-architecture.md

@@ -128,7 +128,6 @@ For trace debugging: `ENGINE_V2_TRACE=1` writes full JSON traces to `engine_trac
 |------|---------|-------------|
 | `Summary` | What a thread accomplished | Conversation insights mission |
 | `Lesson` | Durable learning from experience | Self-improvement mission |
-| `Playbook` | Reusable multi-step procedure | Legacy (superseded by Skill) |
 | `Skill` | Reusable skill with activation metadata and code snippets | Skill extraction mission, v1 migration |
 | `Issue` | Detected problem for follow-up | Self-improvement mission |
 | `Spec` | Missing capability request | Self-improvement mission |
@@ -154,7 +153,7 @@ On each LLM call, two knowledge sources are injected into the system prompt:
 
 ## Skills System
 
-Skills are the v2 replacement for both v1 SKILL.md prompt extensions and v1 playbooks. They provide deterministic, keyword-driven knowledge injection with optional executable code snippets for the CodeAct runtime.
+Skills are the v2 evolution of SKILL.md prompt extensions. They provide deterministic, keyword-driven knowledge injection with optional executable code snippets for the CodeAct runtime.
 
 ### Architecture
 

docs/self-improvement.md

@@ -151,7 +151,7 @@ For Rust bugs in the engine or bridge, the self-improvement thread describes the
 
 ## Fix Pattern Database
 
-A Playbook MemoryDoc maps known trace symptoms to fix strategies:
+A Note MemoryDoc maps known trace symptoms to fix strategies:
 
 | Trace Pattern | Fix Strategy | Location |
 |---|---|---|