## Summary

- Emit `Thread History Viewed` from the shared archive/history opening
path so it covers button, keyboard, and onboarding entry points.
- Emit `Agent Threads Import Clicked` when users open external-agent
thread import from thread history or onboarding.
- Emit `Agent Threads Import Clicked` when users click cross-channel
thread import onboarding.

## Tests

- `cargo fmt --check`
- `git diff --check`
- `./script/clippy -p sidebar`
- `cargo test -p sidebar`

Release Notes:

- N/A