refactor(decopilot): remove dead control-frame wire codec

[pedrofrxncx]

Source: dead-code sweep (C1) of apps/api/src/api/routes/decopilot/ — the area other PRs this tick weren't touching.

What: encodeControlFrame/decodeControlFrame in control-frames.ts have zero callers anywhere in the repo outside their own test file. rg -n "encodeControlFrame|decodeControlFrame" across the whole tree turns up only the definitions and control-frames.test.ts. The CancelBroadcast.publishControlFrame path these were built to serialize for is a documented no-op in NatsCancelBroadcast (publishControlFrame(_userSub, _frame) {}) and in the app-wiring stub in app.ts — the comment on the interface says the durable cancel_requested_at flag is the real correctness path, so the wire codec was never wired up to anything live.

The ControlFrame type/schema stays: it's still the real contract type for the CancelBroadcast.publishControlFrame(userSub, frame: ControlFrame) interface method used in nats-cancel-broadcast.ts, cancel-broadcast.ts, routes.ts, and app.ts. Only the unused JSON encode/decode functions and their now-orphaned test go.

Payoff: removes vestigial code from the old tunnel-based control-frame delivery so a future reader doesn't waste time tracing a codec that's never called.

Net: -39 lines / +0.

Verified:

• rg confirms zero callers of either function outside the deleted test.
• bun run fmt — clean.
• cd apps/api && bunx tsc --noEmit — clean (no type errors from the removal).
• bunx oxlint apps/api/src/api/routes/decopilot/control-frames.ts — 0 warnings/errors.

A reviewer can confirm with: rg -n "encodeControlFrame|decodeControlFrame" apps packages (only doc/PR-body hits expected).

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Summary by cubic

Removes the unused JSON codec for Decopilot control frames to reduce dead code. The codec had no callers; CancelBroadcast.publishControlFrame is a no-op, and the ControlFrame type/schema remains.

Review notes

• Deletes control-frames.test.ts and removes encodeControlFrame/decodeControlFrame from control-frames.ts; keeps the ControlFrame schema/type.
• No runtime behavior change; cancel signaling relies on the durable cancel_requested_at flag.
• Suggested checks: rg -n "encodeControlFrame|decodeControlFrame" shows no callers; typecheck and lint pass.
• Migration: none.

^{Written for commit 3d3d843. Summary will update on new commits.}