Add an advisory “Ready to merge?” summary to the merge dialog

[johannesjo]

Context

Parallel Code already tracks several signals that help a user decide whether a task branch is safe to merge, but they are spread across the title bar, PR link, changed-files panel, and merge validation.

Add a compact summary to the merge dialog so the final decision is visible in one place.

This is inspired by GitHub Code Quality's pull-request quality gates, adapted to Parallel Code's local worktree workflow:
https://docs.github.com/en/code-security/concepts/code-quality/code-quality

Scope

• Show one overall state: Ready, Needs attention, Not ready, or Checking.
• Include these first-party signals:
  • merge safety (conflicts, branch mismatch, committed/uncommitted state)
  • structured task verification
  • GitHub PR check status, when available
• Explain every result with visible text; do not rely on color alone.
• Keep the panel advisory in this first increment.
• Preserve all existing hard merge blockers and merge behavior.
• Derive the presentation from a small, pure, unit-tested readiness model.

Non-goals

• Coverage regression calculations
• Automated code-quality scanning
• New persistence or project settings
• Changing which branches may be merged
• GitHub-only behavior

Acceptance criteria

• The merge dialog displays the overall readiness state before the branch contents.
• Conflicts, detached/mismatched branches, and no committed changes show Not ready.
• Uncommitted changes, failed/blocked verification, pending/failing PR checks, or unavailable verification show Needs attention.
• Passing available checks with a mergeable branch show Ready.
• Loading merge data shows Checking without flashing stale results.
• Existing merge-button enablement remains unchanged.
• Unit tests cover ready, checking, attention, and blocked combinations.