Choose the base ref used to compare the current branch

[felipeorlando]

Problem

MonoCode hardcodes the repository default branch as the comparison/PR base. git_range_context_for resolves the default branch and builds <base>...HEAD; the Changes UI and Create PR affordance do not let the user choose another parent/base.

This is wrong for stacked branches, release branches, and worktrees started from another feature branch.

Proposed behavior

• Show the relationship explicitly as current branch → compare base.
• Add a base-ref picker with the repository default as the initial value and local/remote branches as choices.
• Use the selected base for branch-level changed files/counts, generated PR context, and PR target.
• Preserve the existing working-tree/index view for staging and committing; selecting a branch base must not change checkout state.
• Remember the choice per repository/worktree. A newly created worktree can default to its start-from ref when that metadata is available.

Use merge-base/three-dot semantics so the review shows work introduced by the current branch rather than unrelated changes made on the base since the branch forked.

Existing work checked

• No current upstream issue or PR provides a selectable compare base.
• Closed PR Scope session changes to the session's own baseline hardbeat920/monocode#27 was about session-start filesystem snapshots, not branch-to-parent comparison.
• Orca's diff viewer provides the relevant precedent: default to the worktree's start-from ref and allow comparison against another branch, commit, or base ref: https://www.onorca.dev/docs/review/diff-viewer

Acceptance criteria

• The UI always makes the current branch and selected base visible.
• Changing the base refreshes the branch diff without checking out either ref.
• Generated PR title/body context and target use the same selected base.
• Default-branch behavior remains unchanged until the user selects another base.
• Invalid or deleted refs fall back clearly instead of silently comparing against a different branch.