feat(pair): let Codex lead a pair

[rynfar]

What

Codex can lead a pair again. #638 refused it because a paired Codex lead handed work to its own subagents every time. #642 found and fixed the cause: Pylon's instructions were not reaching Codex at all, so the lead had never been told about the executor or the pair tools.

• isPairLeadSupported on the server and in client-runtime refuses only Antigravity again, so pair_start and the composer's Pair control both accept a Codex thread.
• The internals and user docs say what is true now: for Codex the protocol is the only control, because its collaboration.* tools cannot be removed.

Evidence

Isolated server, fresh Codex thread (codex-cli 0.153.4, GPT-6-Astra) paired with a Claude executor, on a build with #640 and #642, task "brief your executor to create a file, wait, verify":

• One pair_handoff, one pair_await that blocked until the executor finished, the executor's own Claude session wrote the file, the lead verified it.
• Zero collaboration.* calls. Before fix(codex): send Pylon's instructions when a thread starts #642 the same task produced them in 4 of 4 runs.

Verification

• vp test run on the pair and delegation toolkits, client-runtime pair state, the web Pair control tests and RuntimeInstructions.test.ts, on this branch rebased over test(pair): advance the clock by the whole await cap #643: 193 passed. The two lead-support tests were changed first and seen failing.
• Typecheck t3 and @t3tools/client-runtime: 0 errors. Format clean.

Known limits

• One clean run, not a rate. A Codex lead can still choose its own subagents, since nothing prevents it; if that shows up in use, the refusal is two lines to restore.
• Prime Agent, Cursor and OpenCode leads are allowed and still not run end to end.

Part of #622.

Model: Claude Fable 5.1, in Pylon via the Claude Code harness.

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Thread transfer impact

✅ Thread transfer remains within every enforced ceiling.

Provider	Metric	Main baseline	This PR	Impact	PR ceiling
Codex	Total thread wire	14.0 KiB	13.9 KiB	−28 B (−0.2%)	15.1 KiB	✅
Codex	Thread snapshot wire	7.2 KiB	7.2 KiB	−12 B (−0.2%)	7.3 KiB	✅
Codex	Live turn WebSocket wire	6.7 KiB	6.7 KiB	−16 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	14.0 KiB	+39 B (+0.3%)	15.1 KiB	✅
Claude	Thread snapshot wire	7.2 KiB	7.2 KiB	+3 B (+0.0%)	7.3 KiB	✅
Claude	Live turn WebSocket wire	6.7 KiB	6.8 KiB	+36 B (+0.5%)	7.8 KiB	✅
Claude	Live turn WebSocket decoded	58.9 KiB	58.9 KiB	0 B (0.0%)	66.4 KiB	✅
Claude	Live turn messages	9	9	0 (0.0%)	21	✅

Baseline: 463be37 · PR result: 5b6bd14 · 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: 115.6 KiB
• Claude decoded thread snapshot: 116.3 KiB

Updated in place by a trusted workflow. PR artifacts are strictly validated and never executed.