chore: add repo refresh onboarding and CI - #42

Add CI workflow, Biome linting, and doctor preflight script for repo onboarding

• Adds a GitHub Actions CI workflow that runs typecheck, lint, doctor, and fast tests on PRs and pushes to main using Bun 1.3.6 and ffmpeg.
• Introduces src/cli/doctor.ts to validate the local toolchain (Bun, ffmpeg, ffprobe), fixture presence, and writable directories, exiting non-zero on failure.
• Adds biome.json with recommended lint rules and noExplicitAny as an error; updates package.json with lint, typecheck, check, test:fast, and test:ci scripts with explicit 30s timeouts.
• Fixes loadRuleset in src/domain/rules.ts to resolve the ruleset path relative to the module file rather than process.cwd(), and adds validation for required top-level keys.
• Fixes PPM parsing in tests/helpers/image.ts to skip comment lines and strictly validate pixel count; diffRgb.meanChannelDelta now reports per-pixel delta (3× larger than before).
• Behavioral Change: loadRuleset now throws if profiles or white_canvas keys are missing, and resolves config from the module path — callers relying on CWD-relative resolution will break.

^{Macroscope summarized 2e64b0d.}