Add agent_send_async with channel-agnostic callback delivery

[pbranchu]

Summary

• Add agent_send_async tool for non-blocking delegation to other agents (hands)
• Add agent_cancel tool to abort in-flight delegations
• Callback results are delivered through the channel bridge, not a hardcoded voice channel — works on Chat, Voice, Email, and all other channels
• One-turn callback framing: the hand's name appears as the sender (e.g. "workspace-hand wrote: ...") so the agent can present results naturally

Design

When agent_send_async is called:

1. Capture channel context (channel type, sender, thread) from the bridge dispatch
2. Spawn background task to run the target agent
3. Return immediately so the caller can acknowledge the user

When delegation completes:

1. Call inject_async_callback on the kernel handle
2. Send the result to the caller agent with hand attribution
3. Route the agent's formatted response to the original user on the originating channel via send_channel_message

Closes #891.

Test plan

• Chat delegation: message on Chat → delegate → result appears in Chat
• Voice delegation: same flow, result spoken via TTS
• Cancel: agent_cancel aborts in-flight task
• cargo check -p openfang-kernel -p openfang-runtime -p openfang-channels -p openfang-api
• cargo fmt --check

🤖 Generated with Claude Code

[jaberjaber23]

Reviewed and approved. This PR has merge conflicts with recent changes on main. Please rebase onto current main and we will merge it.

[jaberjaber23]

Reviewed and approved. Has merge conflicts with recent changes on main. Please rebase and we will merge immediately.

[pbranchu]

Update needed: channel-agnostic callback delivery

The current implementation delivers async delegation results via a voice-only channel (ASYNC_RESULT_TX). This works for voice sessions but silently drops results for Chat, Email, and all other channels.

Design and implementation plan in #891 — the fix routes callbacks through the channel bridge using the existing per-turn system prompt injection mechanism. Key changes:

• Store minimal channel context (channel type, sender, thread) at delegation time
• On completion, inject callback through BridgeManager::inject_callback()
• Bridge applies channel overrides, system prompt, and routes response to the originating channel
• Removes the voice-specific ASYNC_RESULT_TX side channel

Will update this PR with the fix.