fix(daemon): don't persist browser previews to the transcript

[edwin-zvs]

Browser preview events (SessionEvent::BrowserPreview) carry a full-size Chrome screenshot as a base64 PNG. They were falling through to append_event, so each screenshot was written as a line in transcript.jsonl.

Why this is a problem

• Bloat: the browser tool base64-encodes the raw, full-resolution screenshot (adapter-zarvis/.../tools/browser.rs — no shrink before emit; base64 adds ~33%). Every browser_open / browser_screenshot preview added a 50 KB–500 KB+ line.
• Slow loads: read_transcript reads and JSON-parses every line, and session.transcript ships the whole result over the wire — so each client load (TUI hydration / webui loadTranscript) paid for all those big lines.
• No consumer: after feat(webui): browser preview as top-right terminal overlay + session-scoped, ephemeral thumbnails #162, both clients render previews only from the live broadcast and explicitly skip BrowserPreview during transcript replay. The persisted images were pure dead weight.
• Model leak: agentd_get_transcript (MCP) returns the raw transcript JSON to the model, so persisted previews could dump full base64 screenshots into model context — exactly what BrowserPreview's doc says not to do.

Fix

Skip persistence for BrowserPreview and broadcast to live clients only — the same treatment AgentStatus (also ephemeral live UI state) already gets. Returns before append_event.

The live overlay + matrix-rain wallpaper are unaffected (both fed by the broadcast, not the transcript). Existing transcript.jsonl files are left as-is (not retroactively cleaned).

Tests

• New browser_preview_is_not_persisted_to_transcript: emits a BrowserPreview + a control Message via handle_event, then asserts the transcript contains the Message but not the BrowserPreview.
• Full daemon suite (36) + e2e (web_smoke, tui_smoke, restart, remote_control) green locally.

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