`AgentPanel::open_thread` (called when clicking `@thread` mention links,
pasting thread links, or opening threads via the CLI) was calling
`external_thread` directly, bypassing the de-duplication guards in
`load_agent_thread`. This meant clicking a thread link to the
currently-active thread would create a redundant `ConversationView`
sharing the same `NativeAgent` session. When the old CV was later
evicted from `background_threads`, its `on_release` called
`close_all_sessions`, removing the session and breaking the surviving CV
with "Session not found".

Fix this by routing `open_thread` through `load_agent_thread`, which
checks the active view, retained threads, and background threads before
creating a new `ConversationView`. This prevents the duplicate from
being created in the first place.

Release Notes:

- Fixed a "Session not found" error that could occur after clicking a
thread mention link.