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/background-agent/spawner.ts

@@ -146,7 +146,6 @@ export async function startTask(
       tools: {
         ...getAgentToolRestrictions(input.agent),
         task: false,
-        delegate_task: false,
         call_omo_agent: true,
         question: false,
       },
@@ -231,7 +230,6 @@ export async function resumeTask(
       tools: {
         ...getAgentToolRestrictions(task.agent),
         task: false,
-        delegate_task: false,
         call_omo_agent: true,
         question: false,
       },