refactor: migrate delegate_task to task tool with metadata fixes

- Rename delegate_task tool to task across codebase (100 files)
- Update model references: claude-opus-4-6 → 4-5, gpt-5.3-codex → 5.2-codex
- Add tool-metadata-store to restore metadata overwritten by fromPlugin()
- Add session ID polling for BackgroundManager task sessions
- Await async ctx.metadata() calls in tool executors
- Add ses_ prefix guard to getMessageDir for performance
- Harden BackgroundManager with idle deferral and error handling
- Fix duplicate task key in sisyphus-junior test object literals
- Fix unawaited showOutputToUser in ast_grep_replace
- Fix background=true → run_in_background=true in ultrawork prompt
- Fix duplicate task/task references in docs and comments

src/features/AGENTS.md

@@ -56,7 +56,7 @@ features/
 
 ## ANTI-PATTERNS
 
-- **Sequential delegation**: Use `delegate_task` parallel
+- **Sequential delegation**: Use `task` parallel
 - **Trust self-reports**: ALWAYS verify
 - **Main thread blocks**: No heavy I/O in loader init
 - **Direct state mutation**: Use managers for boulder/session state