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/dynamic-agent-prompt-builder.ts

@@ -242,7 +242,7 @@ ${builtinRows.join("\n")}`
 
   return `### Category + Skills Delegation System
 
-**delegate_task() combines categories and skills for optimal task execution.**
+**task() combines categories and skills for optimal task execution.**
 
 #### Available Categories (Domain-Optimized Models)
 
@@ -296,7 +296,7 @@ SKILL EVALUATION for "[skill-name]":
 ### Delegation Pattern
 
 \`\`\`typescript
-delegate_task(
+task(
   category="[selected-category]",
   load_skills=["skill-1", "skill-2"],  // Include ALL relevant skills — ESPECIALLY user-installed ones
   prompt="..."
@@ -305,7 +305,7 @@ delegate_task(
 
 **ANTI-PATTERN (will produce poor results):**
 \`\`\`typescript
-delegate_task(category="...", load_skills=[], run_in_background=false, prompt="...")  // Empty load_skills without justification
+task(category="...", load_skills=[], run_in_background=false, prompt="...")  // Empty load_skills without justification
 \`\`\``
 }