git-commit-story: add init and Copilot prompt hook

[Dominic Nguyen (datduyng)]

Adds git commit-story init, which registers Copilot CLI hooks for the current repository, and one hook handler.

$ git commit-story init
Registered Copilot hooks in /private/tmp/gcs-e2e/.github/copilot/settings.local.json
$ cat .github/copilot/settings.local.json
{
    "hooks": {
        "userPromptSubmitted": [
            {
                "type": "command",
                "bash": "git commit-story hooks copilot user-prompt-submitted",
                "powershell": "git commit-story hooks copilot user-prompt-submitted"
            }
        ]
    }
}
$ git commit-story hooks copilot user-prompt-submitted
Hello World

settings.local.json is added to .git/info/exclude, so the hooks stay local to the clone. Verified with copilot -p "say hi" in a trusted repo: the hook ran and printed Hello World.

[Dominic Nguyen (datduyng)]

06:02:58.347Z  event_type: "hook.start"
06:02:58.732Z  [rust:hooks] [hook stdout] Hello World
06:02:58.732Z  event_type: "hook.end"
06:02:58.734Z  telemetry (kind: user_message)

Confirmed working