fix: support rollback sprints in local git mode

[numnx]

What changed

• Enable rollback assessment and creation for local Git projects.
• Resolve local default and source branches from local refs, create the rollback commit in an isolated worktree, and skip fetch/push operations.
• Keep local rollback finalization on the existing branch-to-default merge path without enabling pull request monitoring.
• Make shared branch preflight local-only in local Git mode, even when an origin remote is configured.
• Update rollback UI copy, backend/dashboard coverage, and canonical/public documentation.

Why

Rollback sprints previously enforced remote Git mode and assumed every rollback ended in a pull request. Local Git projects need the same rollback safety classification while delivering the rollback by merging the local rollback branch directly into the configured default branch.

Validation

• pnpm run lint
• pnpm run test:backend — 5,494 passed, 7 skipped
• pnpm run test:dashboard — 2,152 passed, 2 skipped
• pnpm run build
• Dashboard responded at http://localhost:4444/
• Real temporary local repository test verifies automatic rollback with no remote configured

Risk / rollback

Risk is concentrated in branch preflight mode selection and rollback ref resolution. Reverting commit 89d3405fb restores the previous remote-only rollback behavior.