feat: use GitHub display name in Co-authored-by trailers (#163)

* feat: use GitHub display name in Co-authored-by trailers

- Add name field to GitHubAuthor type
- Update GraphQL queries to fetch user display names
- Add triggerDisplayName to CommonFields type
- Extract display name from fetched GitHub data in prepareContext
- Update Co-authored-by trailer generation to use display name when available

This ensures consistency with GitHub's web interface behavior where
Co-authored-by trailers use the user's display name rather than username.

Co-authored-by: ashwin-ant <ashwin-ant@users.noreply.github.com>

* fix: update GraphQL queries to handle Actor type correctly

The name field is only available on the User subtype of Actor in GitHub's
GraphQL API. This commit updates the queries to use inline fragments
(... on User) to conditionally access the name field when the actor is
a User type.

🤖 Generated with [Claude Code](https://claude.ai/code)

Co-Authored-By: Claude <noreply@anthropic.com>

* refactor: clarify Co-authored-by instructions in prompt

Replace interpolated values with clear references to XML tags and add
explicit formatting instructions. This makes it clearer how to use the
GitHub display name when available while maintaining the username for
the email portion.

Changes:
- Use explicit references to <trigger_display_name> and <trigger_username> tags
- Add clear formatting instructions and example
- Explain fallback behavior when display name is not available

🤖 Generated with [Claude Code](https://claude.ai/code)

Co-Authored-By: Claude <noreply@anthropic.com>

* feat: fetch trigger user display name via dedicated GraphQL query

Instead of trying to extract the display name from existing data (which
was incomplete due to Actor type limitations), we now:

- Add a dedicated USER_QUERY to fetch user display names
- Pass the trigger username to fetchGitHubData
- Fetch the display name during data collection phase
- Simplify prepareContext to use the pre-fetched display name

This ensures we always get the correct display name for Co-authored-by
trailers, regardless of where the trigger came from.

🤖 Generated with [Claude Code](https://claude.ai/code)

Co-Authored-By: Claude <noreply@anthropic.com>

---------

Co-authored-by: claude[bot] <209825114+claude[bot]@users.noreply.github.com>
Co-authored-by: ashwin-ant <ashwin-ant@users.noreply.github.com>
Co-authored-by: Claude <noreply@anthropic.com>

test/create-prompt.test.ts

@@ -316,7 +316,7 @@ describe("generatePrompt", () => {
 
     expect(prompt).toContain("<trigger_username>johndoe</trigger_username>");
     expect(prompt).toContain(
-      "Co-authored-by: johndoe <johndoe@users.noreply.github.com>",
+      'Use: "Co-authored-by: johndoe <johndoe@users.noreply.github.com>"',
     );
   });