chore(test): move vitest cache to node_modules/.cache

[John-David Dalton (jdalton)]

Summary

• Vitest's cacheDir moves from <repo>/.cache/vitest to <repo>/node_modules/.cache/vitest in the shared vitest base config. The isolated config extends from it, so it picks up the change automatically.
• pnpm install already wipes node_modules/ (and its .cache/ subdir), so this buys us free cache invalidation on every install — no dedicated clean step needed.
• Matches the pattern just applied in ../socket-packageurl-js, ../socket-sdk-js (Cleanup debugFn messages #616), and ../meander.
• Drops **/.cache/ from .gitignore since nothing writes to a top-level .cache/ anymore.

Test plan

• CI runs pnpm lint + pnpm test + pnpm run build.
• Local pnpm install / pnpm test — not run here; the worktree I authored this from had a pre-existing packages/package-builder/build/… workspace-link issue that blocked install. The pre-commit hook's docstring expressly allows --no-verify for local bypass while the pre-push hook continues to enforce security checks, so the commit was made with --no-verify knowing CI will fully verify. Nothing in the diff would be affected by the skipped local check (it's a 2-line config change + gitignore cleanup).

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Note

Low Risk
Low risk: only changes test runner cache location and a gitignore pattern, with no runtime or production code impact.

Overview
Vitest now writes its cache to node_modules/.cache/vitest (instead of a top-level .cache/vitest), relying on pnpm install to naturally invalidate caches by wiping node_modules/.

.gitignore is tightened by removing the broad **/.cache/ ignore since the repo no longer uses a top-level .cache/ directory for Vitest.

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