feat(commands): add /commit command

[DeepamJha]

Closes #757

Description

Adds a new /commit slash command that generates a Conventional Commit
message from the currently staged Git diff.

The implementation follows the approach discussed in #757:

• Checks for staged changes before making an LLM request.
• Reads the staged diff using git diff --cached.
• Reuses the active conversation LLM client (metadata.client) instead of
  creating a new client.
• Uses a dedicated system prompt to generate only a Conventional Commit
  message.
• Returns the generated message without automatically executing
  git commit.
• Handles missing staged changes, empty model responses, and LLM errors
  using the existing message factory.

Type of Change

• Bug fix
• New feature
• Breaking change
• Documentation update

Changeset

• Added a changeset (pnpm changeset) describing this feature for the changelog.

Testing

Automated Tests

• Added commit.spec.tsx
• pnpm build
• pnpm test
• pnpm test:lint

Manual Testing

• Tested with OpenRouter

Verified that:

• /commit warns when no changes are staged.
• /commit generates a Conventional Commit message from the staged diff.
• Model errors are surfaced through the existing error message flow.

Checklist

• Code follows project style guidelines
• Self-review completed
• Documentation updated (if needed)
• No breaking changes
• Appropriate logging added using structured logging (see CONTRIBUTING.md#logging)

(No additional logging was required for this command.)