Allow extensions to emit an open_panel frame at any time, not just as
the action of a command_response. This makes it possible to build
approval gates, secret collection, and freeform user-input prompts
directly inside tool handlers.
Changes:
- extproto: add OpenPanelFromExt wire type
- extensions/manager: route spontaneous open_panel frames to hooks.OpenPanel
- ext/ext.go: add Extension.OpenPanel() SDK method
- tests: TestSpontaneousOpenPanel (manager), TestOpenPanelEmitsCorrectFrame,
TestBlockingToolWaitsForPanelKey, TestBlockingToolDenied (SDK)
- docs/plans: add spontaneous-panel.md design doc
The blocking tool pattern (open panel → block on channel → key event →
tool_result) requires no additional wire changes; it falls out of
standard Go concurrency on the extension side.
Part 3 (intercept timeout for built-in tool gating) is out of scope
and tracked separately.
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