#24812 ci: use Release profile for nightly fast admission

Changelog

Changed: Release admission uses the fleet compile check's Release profile (-c release -Xswiftc -Onone) when the deployed client supports it, and explicitly falls back to the Mac Release compile during rollout.

Why

The old fast path used the Debug profile under the stable Release gate name. Release-only Swift conditionals therefore reached nightly without a fast admission check. The new profile keeps optimization disabled for fast feedback while removing DEBUG conditionals from the checked configuration. The Mac Release job remains the fallback until the private fleet client and recipe are deployed.

Leo's request is to keep building the actual thing continuously; this makes the admission path check the same configuration that the nightly builds.

Validation

• actionlint .github/workflows/cmux-next.yml (only the pre-existing 11 workflow_dispatch input warning)
• git diff --check
• pr-overlap --repo manaflow-ai/cmux --base feat-cmux-next --rev HEAD (no overlapping open PR; the pre-push overlap retry later timed out)
• No native build was run on this host; the fleet and Mac CI lanes own it.

Observed / Not run

• Existing workflow unit tests were started; their long-running suite is still being reconciled separately. No new failure was attributed to this one-line command change.

Agent-Session: 01a16a6a-b5f9-7fe3-8cde-3987dc52cd17

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Summary by cubic

Switches the nightly fast admission check to the Release profile so Release-only Swift conditionals are actually type-checked, instead of silently admitting on a Debug compile under the Release gate name.

• Uses the fleet client's --release flag when available; otherwise falls back to the Mac Release compile with a warning.
• The Mac fallback stays until the private fleet client is deployed with Release support.