Add codiff base to review against a branch's PR/MR base

[KATT]

Summary

Adds a codiff base convenience command that reviews the current branch against the branch its open pull/merge request targets. This is useful for stacked changes where the base is another branch in the stack rather than main.

This ports the idea from Alex's cdf helper into Codiff:

cdf() {
  local base=$(gh pr view --json baseRefName -q .baseRefName 2>/dev/null || echo main)
  git fetch origin "$base" --quiet &
  codiff "origin/$base"
}

Behavior

codiff base:

• Chooses the repository's primary review remote using the same remote priority as PR/MR URL resolution.
• Resolves the current branch's review base:
  • GitHub: gh pr view --json baseRefName --jq .baseRefName
  • GitLab: glab mr view --output json --jq .target_branch
• Falls back to the remote default branch (<remote>/HEAD, then main) when there is no open request or the provider CLI is unavailable.
• Fetches that branch from the remote, then reuses the existing branch review path by opening <remote>/<base>.

The packaged terminal helper delegates codiff base to the Node CLI, so the GitHub/GitLab resolution logic stays in one place instead of being duplicated in shell.

Changes

• Adds base as a positional CLI keyword in bin/arguments.js.
• Adds provider-aware base branch resolution in electron/review-source.cjs.
• Opens the resolved remote branch from bin/codiff.js after a best-effort fetch.
• Routes packaged codiff base through the bundled Node entry point, like --share.
• Documents the command in README.md.
• Covers the new resolver and packaged helper behavior in codiff-cli.test.ts, using explicit resource management for newly added temp resources.

Test plan

• vp check --fix
• vp test core/__tests__/codiff-cli.test.ts
• vp build
• Manual: run codiff base on a stacked GitHub branch
• Manual: run codiff base on a GitLab branch