fix(pi): restore the session's mode after OMP session switches

[andrebrait]

Summary

The shared Pi extension loads in oh-my-pi (OMP), but after /new, /resume or /fork a session keeps the previous session's mode. Pi re-emits session_start on those transitions. OMP emits only the legacy session_switch and session_branch events, which the extension ignored.

This registers those two events next to the existing session_tree handler. All three call the same restoreSessionMode. Nothing else changes, and no OMP-only code is added.

Pi compatibility

Pi 0.85.1 never emits session_switch or session_branch (its changelog says it replaced them with session_start plus a reason), so the new handlers do nothing there. Older Pi versions that still emit session_switch get the same fix. The restore is idempotent and appends no entries.

Verification

• New test: session_switch and session_branch restore the target session's mode, including the default for an empty new session. It fails without the fix.
• node --test tests/*.test.js passes 127 tests, and npm test --prefix pi-extension passes 29.
• I drove both hosts over RPC: set /ponytail lite, prompt, new_session, prompt, switch back to the first session, prompt. The table shows the level injected into each prompt.

Host	Before	After
OMP 18.6.2	lite, lite, lite	lite, full, lite
Pi 0.85.1	lite, full, lite	lite, full, lite

[andrebrait]

A review-only companion PR with the identical diff is at andrebrait#1.

[DietrichGebert]

Thanks! Tested it over RPC with OMP 18.10.3 and pi 0.73.1: on main a new OMP session kept the old level, with this PR it starts at the default and switching back restores the old one; pi behaves the same as before. Merged, will be in the next release.