feat: add get_issue endpoint to retrieve single issue details

Adds functionality to fetch details of a specific GitHub issue by number.
This includes:
- New GetIssueSchema for input validation
- Implementation of getIssue function using GitHub API
- Addition of get_issue tool to available tools list
- Handler for get_issue in CallToolRequestSchema

This allows users to retrieve complete issue information including:
- Issue metadata (title, body, state)
- Associated data (labels, assignees, milestone)
- Timestamps (created, updated, closed)

src/github/schemas.ts

@@ -677,6 +677,12 @@ export const IssueCommentSchema = z.object({
   body: z.string()
 });
 
+export const GetIssueSchema = z.object({
+  owner: z.string().describe("Repository owner (username or organization)"),
+  repo: z.string().describe("Repository name"),
+  issue_number: z.number().describe("Issue number")
+});
+
 // Export types
 export type GitHubAuthor = z.infer<typeof GitHubAuthorSchema>;
 export type GitHubFork = z.infer<typeof GitHubForkSchema>;