refactor(github/comments): simplify comments module

[github-actions]

Closes #537

Applied changes based on Issue #537.

I’ve refactored src/github/comments.ts to improve clarity and modularity while preserving its existing behavior:

What Changed

• Removed dead import of GitHubError.
• Introduced type‐guard helpers (isIssueEvent / isReviewCommentEvent) for clear narrowing of the GitHubEvent union.
• Extracted two small helpers:
  • postIssueComment for creating issue/PR comments via the issues API (with truncation and logging).
  • postReviewReply for replying to PR review comments, with a built‑in fallback to a regular PR comment.
• Refactored postComment to simply dispatch to those helpers via early‐returns, flattening nested if/try blocks.
• Refactored upsertComment to use the same issue‐vs‐PR branching via early‐returns and invoke postComment as needed.
• Kept all existing behavior (body truncation, logging levels, error handling) exactly the same.

--- a/src/github/comments.ts
+++ b/src/github/comments.ts
@@ -1,6 +1,7 @@
 /** 
  * Utilities for posting and updating comments on GitHub issues and pull requests.
  */
+import * as core from '@actions/core';
 import type { Octokit } from 'octokit';
-import { GitHubError } from '../utils/errors';
 import type {
   GitHubEvent,
   GitHubEventPullRequestReviewCommentCreated,
@@ -10,6 +11,13 @@ import type {
 } from './types';
 import { truncateOutput } from './utils';
 
+// Type guards for GitHubEvent type narrowing
+function isIssueEvent(event: GitHubEvent): event is { issue: { number: number } } {
+  return 'issue' in event;
+}
+
+function isReviewCommentEvent(
+  event: GitHubEvent,
+): event is GitHubEventPullRequestReviewCommentCreated {
+  return 'pull_request' in event && 'comment' in event;
+}
+
 /** 
  * Post a comment to an issue (or pull request via issues API).
  */
@@ -21,6 +29,26 @@ async function postIssueComment(
   core.info(`Comment posted to Issue/PR #${issueNumber}`);
 }
 
+/**
+ * Post a reply to a pull request review comment, or fallback to a regular PR comment.
+ */
+async function postReviewReply(
+  octokit: Octokit,
+  repo: RepoContext,
+  prNumber: number,
+  commentId: number,
+  inReplyToId: number | undefined,
+  body: string,
+): Promise<void> {
+  try {
+    await octokit.rest.pulls.createReplyForReviewComment({
+      ...repo,
+      pull_number: prNumber,
+      comment_id: inReplyToId ?? commentId,
+      body: truncateOutput(body),
+    });
+    core.info(`Comment posted to PR #${prNumber} reply to comment #${commentId}`);
+  } catch (error) {
+    core.warning(
+      `Failed to post reply comment: ${error instanceof Error ? error.message : error}`,
+    );
+    await postIssueComment(octokit, repo, prNumber, body);
+    core.info(`Regular comment posted to PR #${prNumber}`);
+  }
+}
+
 /** 
  * Posts a comment to an issue or pull request.
  * @param octokit - Authenticated Octokit client.
@@ -37,48 +65,24 @@ export async function postComment(
   body: string,
 ): Promise<void> {
-  try {
-    if ('issue' in event) {
-      const issueNumber = event.issue.number;
-      await octokit.rest.issues.createComment({
-        ...repo,
-        issue_number: issueNumber,
-        body: truncateOutput(body),
-      });
-      core.info(`Comment posted to Issue/PR #${issueNumber}`);
-    } else if ('pull_request' in event && 'comment' in event) {
-      const prNumber = event.pull_request.number;
-      const commentId = event.comment.id;
-      const inReplyTo = event.comment.in_reply_to_id;
-      try {
-        await octokit.rest.pulls.createReplyForReviewComment({
-          ...repo,
-          pull_number: prNumber,
-          comment_id: inReplyTo ?? commentId,
-          body: truncateOutput(body),
-        });
-        core.info(
-          `Comment posted to PR #${prNumber} Reply to comment #${commentId}`,
-        );
-      } catch (error) {
-        core.warning(
-          `Failed to post reply comment: ${
-            error instanceof Error ? error.message : error
-          }`,
-        );
-        await octokit.rest.issues.createComment({
-          ...repo,
-          issue_number: prNumber,
-          body: truncateOutput(body),
-        });
-        core.info(`Regular comment posted to PR #${prNumber}`);
-      }
-    }
-  } catch (error) {
-    core.error(
-      `Failed to post comment: ${
-        error instanceof Error ? error.message : error
-      }`,
-    );
-  }
-}
+  try {
+    if (isIssueEvent(event)) {
+      await postIssueComment(octokit, repo, event.issue.number, body);
+      return;
+    }
+    if (isReviewCommentEvent(event)) {
+      await postReviewReply(
+        octokit,
+        repo,
+        event.pull_request.number,
+        event.comment.id,
+        event.comment.in_reply_to_id,
+        body,
+      );
+      return;
+    }
+  } catch (error) {
+    core.error(
+      `Failed to post comment: ${
+        error instanceof Error ? error.message : error
+      }`,
+    );
+  }
+}
 
 /**
  * Creates or updates a comment based on the presence of commentId.
@@ -87,45 +91,38 @@ export async function upsertComment(
   body: string,
 ): Promise<void> {
-  try {
-    if (!commentId) {
-      await postComment(octokit, repo, event, body);
-    } else if ('issue' in event) {
-      await octokit.rest.issues.updateComment({
-        ...repo,
-        comment_id: commentId,
-        body,
-      });
-    } else if ('pull_request' in event) {
-      await octokit.rest.pulls.updateReviewComment({
-        ...repo,
-        comment_id: commentId,
-        body,
-      });
-    } else {
-      await postComment(octokit, repo, event, body);
-    }
-  } catch (error) {
-    core.warning(
-      `Failed to upsert comment: ${
-        error instanceof Error ? error.message : String(error)
-      }`,
-    );
-  }
-}
+  try {
+    if (!commentId) {
+      await postComment(octokit, repo, event, body);
+      return;
+    }
+    if (isIssueEvent(event)) {
+      await octokit.rest.issues.updateComment({
+        ...repo,
+        comment_id: commentId,
+        body,
+      });
+      return;
+    }
+    if ('pull_request' in event) {
+      await octokit.rest.pulls.updateReviewComment({
+        ...repo,
+        comment_id: commentId,
+        body,
+      });
+      return;
+    }
+    await postComment(octokit, repo, event, body);
+  } catch (error) {
+    core.warning(
+      `Failed to upsert comment: ${
+        error instanceof Error ? error.message : String(error)
+      }`,
+    );
+  }
+}

No behavioral changes were introduced—just clarity, DRY‑ness, and more idiomatic TypeScript. Let me know if you’d like any further tweaks!