fix(preview): prefer visible browser for new agent leases

[rynfar]

When several Pylon windows can automate a preview, a new agent session can attach to a window that does not own the visible browser tab, losing its current cookies and DOM. Hosts now report local live tabs and visibility; a fresh unbound lease prefers the visible owner of the target thread/tab. Existing live leases stay pinned, and explicit tab IDs with a known owner fail closed if that owner cannot perform the requested operation. Hosts without ownership reports retain legacy selection. This adapts upstream 894d33419dceb366eecda8e5e27c6fc043c3959e (#13064) while preserving Pylon's capability-rich host preference for multi-operation sessions.

The web host reports only tabs with local web contents, clears ownership when contents close, and retries failed focus reports on the next focus/visibility change. A server snapshot alone no longer claims an automation runtime is available.

Validation: 43 focused broker/web-host tests, scoped lint (one pre-existing React refs warning in an unchanged line), and server, web, and contracts typechecks passed. Independent review passed on the frozen diff. No live browser interaction was performed.

Refs #736.

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Project	Deployment	Actions	Updated
pylon-marketing	Ignored	Preview	Sep 24, 2026 8:50am UTC

[github-actions]

Thread transfer impact

✅ Thread transfer remains within every enforced ceiling.

Provider	Metric	Main baseline	This PR	Impact	PR ceiling
Codex	Total thread wire	13.9 KiB	13.9 KiB	+2 B (+0.0%)	15.1 KiB	✅
Codex	Thread snapshot wire	7.2 KiB	7.2 KiB	−9 B (−0.1%)	7.3 KiB	✅
Codex	Live turn WebSocket wire	6.7 KiB	6.7 KiB	+11 B (+0.2%)	7.8 KiB	✅
Codex	Live turn WebSocket decoded	58.0 KiB	58.0 KiB	0 B (0.0%)	66.4 KiB	✅
Codex	Live turn messages	9	9	0 (0.0%)	21	✅
Claude	Total thread wire	13.9 KiB	13.9 KiB	+6 B (+0.0%)	15.1 KiB	✅
Claude	Thread snapshot wire	7.2 KiB	7.2 KiB	+6 B (+0.1%)	7.3 KiB	✅
Claude	Live turn WebSocket wire	6.7 KiB	6.7 KiB	0 B (0.0%)	7.8 KiB	✅
Claude	Live turn WebSocket decoded	58.8 KiB	58.8 KiB	0 B (0.0%)	66.4 KiB	✅
Claude	Live turn messages	8	8	0 (0.0%)	21	✅

Baseline: 47bf17d · PR result: e0f29df · Source CI: success

Scenario and decoded snapshot size

10 historical turns, 5 command tools per turn, 878.9 KiB retained MCP result per historical turn, and a 1.05 MiB retained result in the measured turn.

• Codex decoded thread snapshot: 115.7 KiB
• Claude decoded thread snapshot: 116.3 KiB

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[rynfar]

Adversarial review found that an existing live provider lease could route an explicit tab ID owned by another host back to its pinned host. This head rejects that mismatch without moving or dropping the lease; a regression verifies the rejection and subsequent pinned operation. Synced onto current pylon after #852; 43 focused broker/web-host tests and server, web, contracts typechecks pass. New exact-head CI is running.