Vercel preview paths gate (W8 / #1690)

Stop wasting Vercel preview builds on PRs that don't touch docs/site/.

Before vs after

Scenario	Before	After
PR touches `docs/site/`	deploys	deploys
PR touches only `src/`	deploys (waste)	cancelled
cc-snapshot PR (only `shared/`)	deploys (waste)	cancelled
Override PR (only `package.json`)	deploys (waste)	cancelled
main branch push	deploys	deploys

Mechanism

{
  "git": { "deploymentEnabled": { "main": true } },
  "github": { "silent": true, "autoJobCancelation": true },
  "ignoreCommand": "git diff HEAD^ HEAD --quiet -- ."
}

Project root in Vercel dashboard is set to docs/site/. The ignoreCommand runs from there and exits 0 (skip) when no changes under that path. github.silent suppresses the inline PR comment.

Estimated savings

~85% of Vercel preview builds. Only ~10–15 of ~120 monthly PRs touch docs/site/.