feat(native): add durable Codex thread click actions

[777genius]

Adds a self-contained desktop_thread_v1 click action that opens the supplied Codex thread through an explicitly selected, signature-checked application. The action is stored in notification userInfo, so callback execution does not depend on the sender process or a temporary request file.

The callback lifecycle tracks completion separately from owned child-process cleanup. Bounded background discovery keeps deadlines responsive; expired work cannot issue a new open. Legacy actions retain their codecs and successful command-before-activation order, including the case where a shell exits before its ordinary background helper.

Stacked on #153 (feat/agent-notify-native-protocol). This is the native portion of phase 3. Installer activation and the Go/MCP consumer are subsequent integration work.

Validation:

• macOS Swift suite: 82 tests, 2 opt-in tests skipped, zero failures.
• Both opt-in real child-process tests passed separately, covering timeout/reaping and leader-exits-first natural completion/cancellation.
• Independent source review and follow-up review verified the exact patch; all reported findings are resolved in this scope.
• Installed notification clicks, cold start and queued upgrade/rollback remain product qualification gates. An accepted open request is not visible-target confirmation.

[coderabbitai]

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