Add stop and kill actions for orchestration pills

[advait-m]

Description

Adds Stop and Kill actions to orchestration pill overflow menus. Stop cooperatively cancels running child agents, while Kill tears down the child pane/conversation and tombstones its run ID so late events are dropped. Kill also sends one final synthetic Cancelled lifecycle event to the parent before tombstoning so the parent is informed that the child was killed.

Linked Issue

N/A

• The linked issue is labeled ready-to-spec or ready-to-implement.
• Where appropriate, screenshots or a short video of the implementation are included below (especially for user-visible or UI changes).

Testing

Skipped additional checks before creating this PR per request. Earlier validation on this branch:

• cargo fmt

• cargo test -p warp --lib orchestration_events -- --nocapture

• cargo test -p warp --lib orchestration_event_streamer -- --nocapture

• cargo check -p warp

• I have manually tested my changes locally with ./script/run

Screenshots / Videos

Not included.

Agent Mode

• Warp Agent Mode - This PR was created via Warp's AI Agent Mode

CHANGELOG-OZ: Added Stop and Kill controls for orchestration child-agent pills.

Co-Authored-By: Oz oz-agent@warp.dev