feat: delegate tasks to external AI CLIs (Claude, Gemini)

[priyanshujain]

Summary

• Add delegate_task tool that delegates complex tasks to external AI CLIs (Claude, Gemini, Codex) with user approval
• Auto-detects installed CLIs via exec.LookPath at registration time — no config needed
• Supports sync and async (background) execution with check_task for retrieving results
• Async mode uses streaming output with throttled progress notifications (max 1 per 30s)
• Structured spec support: steps, output format, and max budget (Claude --max-budget-usd)
• In-memory TaskTracker with max 3 concurrent background tasks
• Strips CLAUDECODE env var for Claude CLI to avoid nested session detection

New files

File	Purpose
agent/tools/agent_runner.go	CLI detection + execution
agent/tools/agent_stream.go	Streaming NDJSON runner for progress
agent/tools/delegate_task.go	delegate_task tool (sync/async, approval, specs)
agent/tools/task_tracker.go	In-memory background task state
agent/tools/check_task.go	check_task tool (read-only)

Modified files

File	Change
agent/tools/prompt.go	Conditional "Task Delegation" system prompt section
internal/cli/chat.go	Register delegate_task + check_task in CLI
channel/telegram/session.go	Register delegate_task + check_task in Telegram

Test plan

• 59 new tests, all passing with -race detector
• Streaming path tested via mock StreamRunnerInterface
• Progress throttling verified (50 rapid events → 1 notification)
• Async approval, denial, timeout, max concurrent all tested
• Edge cases: malformed JSON, empty task, unknown agent, no agents available
• Integration tests for real Claude/Gemini CLIs (gated with t.Skip)