refactor(mr view): switch from Notes API to Discussions API

Summary

Switch glab mr view --comments from using the Notes API to the Discussions API. This change preserves discussion thread structure and lays the groundwork for future features like filtering by resolved/unresolved status and resolving discussions.

Changes

• Replace listMRNotes with listMRDiscussions
• Update output format to show threaded discussions with note IDs
• Display resolution status (✓ resolved / ⚠️ unresolved) for threads
• Indent replies within discussion threads
• Update all tests to work with Discussion objects
• Flatten discussions to notes for raw/JSON output compatibility

New Output Format

The new format shows:

• Thread [#<note-id>] for multi-note discussions
• Individual notes for non-threaded comments
• Resolution status for resolvable threads
• Proper indentation for replies within threads

Example

Thread [#3107030349] ✓ resolved
  @phikai commented 2026-02-23
  Can you review this?

    @phikai replied 2026-02-25
    Back to you!

    @reviewer replied 2026-02-25
    LGTM!

Why This Change?

This is the first MR in a stack of three:

1. This MR: Switch to Discussions API (foundation)
2. Next MR: Add --resolved and --unresolved filtering flags
3. Final MR: Add glab mr note resolve/unresolve commands

The Discussions API is necessary because:

• Resolution state is tracked at the discussion level, not individual notes
• Thread structure is preserved (original comment + replies)
• Note IDs can be used to identify and resolve discussions

Test Plan

• All existing tests updated and passing
• glab mr view --comments shows discussions with thread structure
• JSON output includes full Discussion objects
• Raw output flattens discussions to notes (backwards compatible)