Add lightweight update_plan tool

[CalvinSturm]

Summary

Adds a lightweight side-effect-free update_plan tool so LocalAgent can track and display multi-step work during an active run.

What changed

• Added an update_plan builtin

• Supports plan item states:

  • pending
  • in_progress
  • completed

• Rejects plans with more than one in_progress item

• Keeps bounded in-memory plan state on the agent

• Emits PlanUpdated events

• Adds compact log summaries for plan updates

• Renders the current plan in the chat TUI when present

• Renders plan updates in the standalone event tail

Why

LocalAgent now has much better shell lifecycle behavior after recent PRs:

• shell timeout + cleanup
• head/tail output truncation
• default shell timeout policy
• live shell output in the TUI tail

The next major opencode-style UX gap is structured task visibility. This PR gives the agent and user a shared view of the current plan without introducing heavy DAG/taskgraph orchestration.

Scope boundaries

This PR intentionally does not:

• touch taskgraph.rs
• add multi-run DAG orchestration
• change provider behavior
• change Docker timeout behavior
• add process-tree cleanup
• redesign the TUI

Validation

• cargo fmt --check
• cargo clippy --all-targets -- -D warnings
• cargo test --lib
• cargo test --test tool_call_accuracy_ci
• cargo test --test artifact_golden
• python scripts/ci_release_readiness.py
• cargo test

All validation passed locally.

Follow-ups

• Persist compact plan summaries in run artifacts if useful
• Add richer TUI styling for plan state transitions
• Add automatic final-plan summary in task closeout
• Robust process-tree cleanup
• Docker shell timeout enforcement