refactor: replace magic number with GITHUB_ACTIONS_BOT_ID constant

- Create new constants file at src/github/constants.ts with GITHUB_ACTIONS_BOT_ID and GITHUB_ACTIONS_BOT_LOGIN
- Replace all occurrences of magic number 41898282 with the constant
- Update imports across source and test files to use the new constants
- Add comment in action.yml referencing the constants file for the default bot_id value

This improves code maintainability by centralizing the GitHub Actions bot ID definition.

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

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

test/mockContext.ts

@@ -9,6 +9,7 @@ import type {
   PullRequestReviewEvent,
   PullRequestReviewCommentEvent,
 } from "@octokit/webhooks-types";
+import { GITHUB_ACTIONS_BOT_ID } from "../src/github/constants";
 
 const defaultInputs = {
   prompt: "",
@@ -18,7 +19,7 @@ const defaultInputs = {
   branchPrefix: "claude/",
   useStickyComment: false,
   useCommitSigning: false,
-  botId: "41898282",
+  botId: String(GITHUB_ACTIONS_BOT_ID),
   allowedBots: "",
   trackProgress: false,
 };