[User Story] As a user, I want `git-copilot graph` to display an interactive commit history

[coderabbitai]

User Story

As a developer investigating a bug,
I want to run git-copilot graph and navigate the commit history interactively,
So that I can identify which commit introduced a regression.

Acceptance Criteria

• Renders a branch-aware commit graph in the terminal (ASCII or Blessed widgets)
• Supports j/k navigation through commits
• Pressing Enter on a commit shows its diff and message
• / key opens an inline search prompt to filter commits by message or author
• --limit <n> flag limits the number of commits shown (default 50)
• Works offline (reads from local git history only)

Backlink: #1