fix(cli): disable Freebuff chat logo sheen

[c8dhjp4tyv-bit]

Summary

• disable the animated chat-header logo sheen in Freebuff builds
• keep the existing animation for standard Codebuff builds
• add focused regression coverage for both product paths

Fixes #1035

Validation

• focused animation tests: 3 passed
• related queue tests: 4 passed
• full CLI suite: 2,367 passed, 9 skipped; existing checkout/environment failures remain (34 failed tests and 32 harness errors)
• CLI typecheck: existing missing tar and react-dom/server declarations; no errors from changed files
• Prettier check for changed files: passed
• git diff --check: passed

[codebuff-team]

Nice, minimal fix. The extraction of shouldAnimateChatHeader into its own pure function in cli/src/utils/chat-header-animation.ts is the right call — it's independently testable and keeps chat-header.tsx unchanged in structure (single line diff). Tests in chat-header-animation.test.ts cover the three meaningful branches (Freebuff-off, Codebuff-on, and inactive-in-either-case), which is proportionate to the size of the change.

A couple of things a maintainer will likely want confirmed before porting:

1. Confirm IS_FREEBUFF is genuinely a build-time/product flag already used elsewhere in cli/src/utils/constants.ts — the diff assumes it exists and is already threaded through chat-header.tsx, which it is per the import list, so this looks consistent with existing conventions rather than introducing a new product branch.
2. The linked issue (Can the FREEBUFF flashing effect displayed by CLI be turned off? It is currently causing the input method to flicker #1035) presumably describes the flicker/visual bug — worth double-checking the fix is actually disabling the right animation and not just papering over a timing bug in useSheenAnimation itself that could resurface elsewhere.

Overall this is a clean, scoped, well-tested change that should port cleanly by hand.

[c8dhjp4tyv-bit]

Verified both points against the current tree:

1. IS_FREEBUFF is the existing Freebuff build-time product flag in cli/src/utils/constants.ts; it is injected via --define/FREEBUFF_MODE and is already the flag imported by chat-header.tsx.
2. Issue Can the FREEBUFF flashing effect displayed by CLI be turned off? It is currently causing the input method to flicker #1035 is specifically about the flashing Freebuff logo causing input-method flicker, and its discussion explicitly asks to turn off the LOGO animation. This PR disables only the chat-header sheen for Freebuff while leaving Codebuff and inactive-header behavior unchanged.

So I don't see a broader useSheenAnimation timing bug that this change is masking; the scoped behavior matches the reported request.