fix(session): make teardown idempotent with handle-based cleanup

[forketyfork]

Summary

• make SessionState.deinit clean up shell, stream, and terminal based on handle presence instead of the spawned flag
• keep SIGTERM guarded by spawned and !dead, but always deinit and null out owned handles
• apply the same handle-based cleanup pattern in resetForRespawn

Why

This makes teardown idempotent when lifecycle flags and handles drift out of sync, avoiding stale-resource deinit crashes during close/relaunch paths.

Validation

• zig build
• zig build test
• just lint

[Copilot]

Pull request overview

This PR refactors the SessionState cleanup logic in deinit and resetForRespawn to make teardown idempotent by cleaning up resources based on handle presence rather than the spawned lifecycle flag. The SIGTERM signal is still guarded by spawned and !dead, but resource deinitialization now happens unconditionally when handles exist.

Changes:

• Modified deinit to clean up shell, stream, and terminal based on handle presence rather than spawned flag
• Modified resetForRespawn to use the same handle-based cleanup pattern
• Lifecycle flags (spawned, dead, cwd_settled) are now reset unconditionally outside handle checks

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