[fingerprint] save db to pushed branch by default

[mmichels-brex]

Why

Fixes #339.

Fingerprint DB entries were only saved to the repository default branch by default. That breaks stacked pull requests because a PR targeting feature-a can only restore caches from feature-a or the default branch, so entries created from pushes to feature-a were missing unless users explicitly configured saving-db-branch.

How

• resolve the default fingerprint DB save branch to the current branch on push events
• keep explicit saving-db-branch as the highest-priority override
• apply the same behavior to both fingerprint and preview-build
• add tests for the shared branch resolver and both save paths
• document the new default behavior and the stacked-branch workflow requirement

Test Plan

• pnpm exec jest --runInBand --watchman=false --forceExit
• node ncc.js