Add live progress streaming for background sub-agents

[Ghislain89]

Describe the feature or problem you'd like to solve

Feature request: Richer agent observability for background sub-agents.

Proposed solution

When running multi-phase orchestrator agents (e.g., feature lifecycle with plan → implement → deliver → review), /tasks only shows tool call count and elapsed time. There is no visibility into what the agent is currently doing — which subtask, which phase, what file it is working on. A live activity log or structured progress stream from background agents would dramatically improve the developer experience for long-running agentic workflows.

Proposed Solution Direction

1. Agent Status Events (lightweight)

Allow background agents to emit structured status events via a report_progress tool (analogous to report_intent):

report_progress({ phase: "implement", subtask: "OEA-1235", detail: "Writing domain tests" })

These events would surface in /tasks as a "last activity" column and in timeline via Ctrl+O.

2. Streaming Timeline for Background Agents

/tasks <agent_id> could open a live tail view showing the agent's timeline events in real-time (tool calls, file edits, shell commands) — similar to how docker logs -f works. Press q to detach without stopping the agent.

3. Structured Progress Protocol

For orchestrator-style agents, define a progress protocol where agents can report:

• Current phase (plan/implement/deliver/review)
• Subtask progress (3/6 done)
• Active file being edited
• Last tool call summary

This could be rendered as a compact progress bar in the CLI:

[orchestrate-ABC-1234] Phase: implement ██████░░░░ 3/6 subtasks | Working on: Sample.java

4. /tasks --watch mode

A polling mode that auto-refreshes the tasks overview every N seconds, showing tool call count delta and last activity for each agent.

Impact

Long-running orchestrator workflows (10-70+ min) currently feel like a black box. Any of these options would let developers stay informed without manually polling or asking the main agent to check.