feat: implement note mode for general codebase commenting

[hay-kot]

Summary

Implements note mode (#5) - a feature for adding comments to any files in the codebase without requiring diff or review context.

Features

• Multiple note sets: Organize notes for different purposes (security audit, refactoring, etc.)
• Persistent storage: Auto-saves to .diff-review/notes/ with JSON format
• Session restore: Automatically restores note mode state on Neovim startup
• Works anywhere: Comment during normal editing, no special layout required
• Consistent UI: Reuses existing keymaps and visual styling from diff review

Implementation

Core Modules (5 new files)

• notes.lua - In-memory storage with set management
• note_persistence.lua - JSON persistence layer
• note_mode.lua - Mode state management and auto-commands
• note_ui.lua - Virtual text rendering with separate namespace
• note_actions.lua - User interaction handlers (add/edit/delete)

Commands (6 new)

• :DiffReviewNoteEnter [set_name] - Enter note mode
• :DiffReviewNoteExit - Exit note mode
• :DiffReviewNoteToggle [set_name] - Toggle mode
• :DiffReviewNoteClear - Clear all notes in current set
• :DiffReviewNoteList - List and switch note sets
• :DiffReviewNoteSwitch <set_name> - Switch to different set

Configuration

require('diff-review').setup({
  notes = {
    default_set = "default",
    auto_restore = true,
  },
})

Testing

• 30 unit tests (21 for notes module, 9 for persistence)
• All tests passing
• Covers CRUD operations, persistence, and set management

Documentation

• Added comprehensive README section
• Usage examples and workflows
• Storage structure documentation
• Configuration examples

Changes

• 10 files changed, 1909 insertions(+)
• No breaking changes
• Fully isolated from existing diff review functionality

Demo Workflow

" Start note mode for security audit
:DiffReviewNoteEnter security-audit

" Navigate files and add comments
<leader>c " Add comment at cursor
<leader>v " View all comments

" Switch to different note set
:DiffReviewNoteSwitch refactoring

" Notes auto-save and persist across sessions

Closes #5