fix: use ANTIGRAVITY_VERSION in ANTIGRAVITY_HEADERS to prevent version mismatch

The ANTIGRAVITY_HEADERS constant was using a hardcoded version string (1.104.0)
instead of the ANTIGRAVITY_VERSION constant (1.15.8), causing "This version of
Antigravity is no longer supported" errors.

Changes:
- Moved ANTIGRAVITY_VERSION declaration before ANTIGRAVITY_HEADERS
- Changed ANTIGRAVITY_HEADERS["User-Agent"] to use template literal with ANTIGRAVITY_VERSION
- Added JSDoc comments explaining the single source of truth pattern
- Added reference to issue #324

This ensures version consistency across all components and prevents future
version drift issues.

Fixes #324