Skip the local existence check for a remote project's tied root

[shenghsi]

Summary

• historical_thread_belongs_to_panel (landed in v0.11.3, PR Fix two ways a historical Agent Thread session can go missing #264) started requiring a tie override's target directory to exist before trusting it, so a session tied to a worktree later deleted falls back to its natural root instead of disappearing from every panel.
• Path::exists only ever inspects the calling process's own filesystem. For a remote (SSH) project, the tied root is a path on the remote host, not this machine, so the check always reported a perfectly valid remote tie as "gone" -- silently discarding every retie for remote-project users, a regression v0.11.3 shipped.
• The check is now skipped for remote projects, restoring the pre-v0.11.3 (unconditional-trust) behavior there. Local projects keep the new existence check.

Test plan

• cargo test -p agent_threads --lib (295 passed)
• Added historical_thread_belongs_to_panel_trusts_a_remote_tie_it_cannot_locally_stat, which reties a thread to a path that cannot exist locally and asserts a remote-project lookup still trusts it
• ./script/clippy -p agent_threads
• cargo fmt --all -- --check

Release Notes:

• Fixed a regression where a retied Agent Thread session in a remote (SSH) project could stop appearing in its History panel