Let agents follow a thread after a mention

[austin-e]

What this changes

Adds an optional conversation setting for an agent: Stay in thread after a mention.

When enabled, an authorized @mention opens a one-hour follow window for that one thread. Later messages in that thread are delivered to the agent as context without another @mention. Messages in every other thread and elsewhere in the channel are ignored.

This does not make the agent reply to every message. It only keeps the agent informed. The agent still decides when a response is useful.

Where to change it

In the agent editor, open Advanced settings and choose either:

• Mention for each message (the existing default)
• Stay in thread after a mention

Safety limits

• Existing agents retain mention-only behavior unless changed.
• Only an already-authorized author can open a followed thread.
• Follow state is kept separately per channel and expires after one hour of inactivity.
• It is in memory only, so an agent restart forgets old followed threads rather than listening more broadly.
• At most 100 active threads are retained; the oldest one is removed first.
• An agent leaving a channel loses all followed threads in that channel.

Implementation

• Adds thread-follow as a harness subscription mode.
• Locally filters the relay's broader delivery so only direct mentions or messages in an active followed thread reach the agent queue.
• Persists the setting through Desktop records, API requests, snapshots, and runtime restart detection.
• Adds the agent-editor control and explanatory copy.

Verification

Passed:

cargo test -p buzz-acp                 # 534 passed
pnpm --dir desktop typecheck
pnpm --dir desktop lint
cargo fmt --check
git diff --check

cargo test --manifest-path desktop/src-tauri/Cargo.toml could not run in this environment because CMake is not installed and audiopus_sys needs it to build bundled Opus.

[austin-e]

Closing for local product testing before review. I will reopen after the interaction has been tried.

[austin-e]

Reopened after local validation.

I also fixed a missing test import found when running the full Desktop test suite.

Final verification:

• cargo test -p buzz-acp: 534 passed
• pnpm --dir desktop typecheck
• pnpm --dir desktop lint
• cargo test --manifest-path desktop/src-tauri/Cargo.toml: 1,477 passed, 13 intentionally ignored
• cargo fmt --check and git diff --check

[austin-e]

Closing again so the updated thread-follow behavior can be tried locally before review.