fix: gracefully handle stale ctx after session replacement

[RobGilto]

Problem

When running pi -p "prompt", the extension throws an error after the response is delivered:

Extension error (.../pi-hooks.ts): This extension ctx is stale after session replacement or reload.
Do not use a captured pi or command ctx after ctx.newSession(), ctx.fork(), ctx.switchSession(),
or ctx.reload().

The -p flag causes pi to create a session, run the prompt, then replace/reload the session. The extension's event handlers still hold a reference to the old ctx and try to use it after replacement.

Fix

Add a safeHandler wrapper in hook-context.ts that catches stale-context errors and returns silently. When a session is replaced mid-handler, the old session no longer exists — there is nothing useful the handler can do, so silently returning is the correct behavior.

All 9 pi.on() registrations across 5 hook modules are wrapped:

• session-hooks.ts — session_start, session_shutdown
• prompt-hooks.ts — input, before_agent_start
• compact-hooks.ts — session_before_compact, session_compact
• stop-hooks.ts — agent_end
• tool-hooks.ts — tool_call, tool_result

Testing

# Before fix: prints response + error
pi -p "hello world"
# => Hello! 👋 ...
# => Extension error (...): This extension ctx is stale...

# After fix: clean output
pi -p "hello world"
# => Hello! 👋 ...