fix(coding-agent): preserve session-start notifications until UI attach

[Diyoncrz18]

Summary

• Preserve extension notifications emitted during the initial session_start bind until the first eligible extension-UI client can receive them.
• Deliver retained startup notifications exactly once to the selected public client without changing normal live notification fanout.
• Prevent the supervisor's internal worker subscription from consuming startup notifications before the public attach response and snapshot are usable.

Root cause

Extensions are bound before a daemon UI client is attached. A session_start handler that called ctx.ui.notify() therefore emitted an extension_ui_request into an empty recipient set. The initial fix retained notifications, but its capture window extended beyond the initial bind, ordinary last-client detach could clear the queue, and a supervisor subscription could release the queue before a public UI client was ready.

Solution

• Capture notify requests only while the initial extension bind is running.
• Retain up to 100 startup notifications in deterministic order, dropping the oldest entry when the limit is reached.
• Keep the queue across temporary detach and clear it only when its lifecycle is invalidated, such as session close or runtime replacement.
• Select the first eligible public extension-UI client as the startup replay recipient, with fallback if that client disconnects or loses eligibility.
• Delay worker extension-UI synchronization until the public attach response is usable and any chunked snapshot has completed.
• Preserve queued frames through snapshot streaming and backpressure, while keeping ordinary post-attach notifications on the existing multi-client broadcast path.

Only fire-and-forget notify requests participate in startup retention. Modal requests such as confirm, select, input, and editor retain their existing response and cancellation behavior. Clients without the per-session extension_ui capability neither receive nor consume retained notifications.

Compatibility

This change is backward-compatible. It only changes internal retention, ordering, and routing of the existing extension_ui_request notification. It adds no daemon command, event type, response field, or startup requirement, so DAEMON_PROTOCOL_VERSION and DAEMON_SCHEMA_REVISION remain unchanged.

Validation

• Issue Daemon drops session_start extension notifications before first client attach #1032 regression suite: 10 tests passed.
• Combined regression, extension-binding, and daemon-connection suites: 77 tests passed.
• Snapshot catch-up regression suite: 9 tests passed.
• Focused daemon-mode and supervisor/backpressure scenarios passed.
• npm run check passed, including formatting, linting, type checking, installer rendering, and browser smoke validation.
• git diff --check passed.

The complete supervisor monitor suite passed 44 tests on Windows; one unrelated Unix-socket test could not create its socket path because Windows returned EACCES. The affected supervisor routing and backpressure cases pass when run directly.

Issue

Fixes #1032

Note

Preserve daemon session-start extension UI notifications until a UI-capable client attaches

• Extension UI notify events emitted during session_start were previously lost if no UI-capable client was attached yet; they are now buffered and replayed once to the first eligible client after attach and snapshot streaming complete.
• Buffering is capped at 100 notifications per session (MAX_PENDING_EXTENSION_UI_NOTIFICATIONS) to bound memory use, and delivery is deferred past backpressure.
• A single recipient is selected per session; detaching that client clears the recipient lock so another can be chosen. Subsequent extension rebinds do not re-open the capture window.
• shouldSendDaemonOutboundToClient now gates notify-type extension_ui_request messages on per-session UI capability, so non-UI clients no longer receive them.
• A regression test suite in 1032-session-start-notification.test.ts covers capture, single-recipient replay, backpressure, rebind, and bounded buffering.

^{Macroscope summarized 39da919.}