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/agents/AGENTS.md

@@ -68,11 +68,11 @@ agents/
 ## TOOL RESTRICTIONS
 | Agent | Denied Tools |
 |-------|-------------|
-| oracle | write, edit, task, delegate_task |
-| librarian | write, edit, task, delegate_task, call_omo_agent |
-| explore | write, edit, task, delegate_task, call_omo_agent |
+| oracle | write, edit, task, task |
+| librarian | write, edit, task, task, call_omo_agent |
+| explore | write, edit, task, task, call_omo_agent |
 | multimodal-looker | Allowlist: read only |
-| Sisyphus-Junior | task, delegate_task |
+| Sisyphus-Junior | task, task |
 | Atlas | task, call_omo_agent |
 
 ## PATTERNS
@@ -85,5 +85,5 @@ agents/
 ## ANTI-PATTERNS
 - **Trust reports**: NEVER trust "I'm done" - verify outputs
 - **High temp**: Don't use >0.3 for code agents
-- **Sequential calls**: Use `delegate_task` with `run_in_background` for exploration
+- **Sequential calls**: Use `task` with `run_in_background` for exploration
 - **Prometheus writing code**: Planner only - never implements