Add explicit review target selection

[dbrosio3]

Summary

• keep review.target_branch as the configured default while detecting ambiguous review bases
• offer an interactive review-target selector for stale targets, existing destination branches, and stacked branches
• support arrow-key or numeric selection, custom refs, and the one-push pushgate.review-target=<ref> override
• use the selected target consistently for deterministic checks, plugins, policies, and local AI review
• print the selected review target and resulting review/scan ranges in the transcript
• validate one-push overrides and preserve terminal input around escape sequences

Why

Choosing the wrong comparison base can produce an inflated changed-line count and incorrectly block a push through ai.max_changed_lines. This PR makes the comparison target explicit when local Git state offers multiple plausible bases.

Related investigation: https://app.notion.com/p/39f59347eba68032bb83e9fdafe8cad5

Developer impact

Unambiguous pushes continue using the configured target without prompting. Ambiguous interactive pushes ask the developer to choose a target; ambiguous non-interactive pushes fail closed with override guidance. Pushgate does not fetch or mutate refs during selection.

Validation

• pnpm run typecheck
• 26 focused tests across workflow-run-plan, terminal, pre-push-stdin, and warning-confirmation
• pnpm run bundle
• git diff --check