Render a dimmed ✓ instead of blank space for deselected checkboxes

[shouze]

Context

Deselected repo/extract checkboxes currently render as a blank space (" ") instead of an inactive checkmark, at three sites in src/render.ts (repo summary line, repo row, extract row). This is visually inconsistent and, once mouse support lands, leaves no visible clickable target for deselected items.

Part of #166. Independent — can be implemented/merged before the other sub-issues.

Solution

• Replace group.repoSelected ? pc.green("✓") : " " (and the two analogous extract/summary variants) with a dimmed inactive checkmark, e.g. pc.dim("✓"), at all three sites.
• Keep column width/alignment identical (the dimmed ✓ occupies the same single character width as the current space, so no layout changes needed elsewhere).

Acceptance Criteria

• Deselected repos and extracts show a dimmed/grey ✓ instead of blank space.
• Selected items keep the existing bright green ✓.
• No layout shift: column alignment for repo names / match counts / extract paths is unchanged.

Definition of Done

• bun test passes, including updates to existing unit tests in src/render.test.ts (or equivalent) if they assert on the literal rendered checkbox string.
• bun run lint passes.
• bun run format:check passes.
• bun run knip passes (no unused exports/imports).
• bun run build.ts compiles successfully.
• Manual visual check in a terminal (screenshot optional) attached to the PR.
• Documentation updated if the keyboard-shortcuts/legend reference describes the checkbox appearance.