chore(engine): remove legacy Playbook doc type, superseded by Skill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

src/bridge/store_adapter.rs

@@ -202,7 +202,6 @@ fn doc_workspace_path(doc: &MemoryDoc) -> String {
     let type_dir = match doc.doc_type {
         DocType::Summary => "summaries",
         DocType::Lesson => "lessons",
-        DocType::Playbook => "playbooks",
         DocType::Issue => "issues",
         DocType::Spec => "specs",
         DocType::Note => "notes",