Add code review workflow with diff viewer and file-by-file navigation

[VineethReddy02]

Summary

• Replace auto-PR behavior with a human-in-the-loop review workflow: agents commit locally instead of pushing/creating PRs, and users review changes before publishing
• Add DiffViewer component with file-by-file navigation, agent summary display, and three sub-views (diff, files, commits)
• Auto-switch to diff review view when a task completes
• Add code review modal with "Approve & Create PR" and "Request Changes" actions
• Add targeted follow-up that pre-fills the prompt with file-specific context

New Review flow:

1. Agent finishes task -> commits locally, writes .hermes/summary.md
2. SessionDetail auto-switches to diff view (no keypress needed)
3. User reviews: browse files with [n]/[p], view all with [a], switch tabs with [1]/[2]/[3]
4. User decides:
   - [c] -> Code Review modal -> [a] Approve & Create PR (pushes + creates PR + opens in browser)
   - [c] -> Code Review modal -> [x] Request Changes (resumes agent with feedback prefix)
   - [f] -> Follow-up (resumes with file-specific context, e.g. "Regarding changes in src/foo.ts: ")

Also includes:

• Fix g key conflict between DiffViewer (scroll-to-top) and SessionDetail (git switch)
• New diffParser module with 6 tests for splitting unified diffs into per-file chunks