Render gate: tighten the loose agy prompt marker (AGY_MARKER = /^> /) #1474

What

The agy (Gemini CLI) render-gate profile keys its prompt detection on AGY_MARKER = /^> / — any line beginning with "> " is treated as the composer prompt. That's a loose match: agy's profile was derived empirically (color-keyed placeholder palette) and this marker is the weakest link, since "> " is common in ordinary output (quoted text, diffs, shell transcripts, agent prose).

Tighten it so the marker identifies the actual agy composer prompt rather than any "> "-prefixed line — e.g. anchor on additional profile signals (cursor row, placeholder-fg palette, region boundary) already used elsewhere in the agy classifier.

Why

A too-loose prompt marker is a false-clean risk: if a "> " line in agy's output is mistaken for an empty composer, the gate could classify CLEAN and deliver onto a busy screen — the exact corruption the mailbox-first design rules out. It hasn't bitten in practice, but it's the least-grounded part of the agy profile and worth hardening while the fixture-capture tooling is fresh.

Notes

• agy classification is the least-covered of the three profiles (Gemini skips non-blockingly when unauthenticated), so a captured real-agy fixture across states (idle / draft / trust-dialog / quoted-output) should back the tightening.
• Distinct from Serializer convergence: route mailbox write edge through submitToSession (serialize gated deliveries vs interrupt/escape) #1365 (serializer convergence) — that bullet in the review pairs them, but this is specifically about the agy prompt marker, not the interrupt-vs-mailbox lock.
• Refs afx send: mailbox-first delivery — persist, gate on rendered-empty, never force-inject #1313 (PR [Spec 1313] afx send: mailbox-first delivery (never force-inject) #1330), Technical Debt / Follow-up sections of codev/reviews/1313-afx-send-mailbox-first-delivery.md.