fix: improve task naming prompt to avoid URL content guessing (#21151)

Previously, when a user created a task with a URL-only prompt (e.g.,
`Let's work on https://github.com/coder/coder/issues/21138`), the LLM
would hallucinate what the URL content might be about - generating names
like "Fix GitHub Actions workflow issue" when the actual issue was
unrelated.

Add examples to the task naming system prompt showing expected behavior
for GitHub issue and PR URLs, teaching the model to use visible URL
parts (repo name, issue/PR number) rather than guessing content.

Co-authored-by: blink-so[bot] <211532188+blink-so[bot]@users.noreply.github.com>

coderd/taskname/taskname.go

@@ -71,6 +71,18 @@ Prompt: "Set up CI/CD pipeline" →
 	"task_name": "setup-cicd"
 }
 
+Prompt: "Work on https://github.com/coder/coder/issues/1234" →
+{
+	"display_name": "Work on coder/coder #1234",
+	"task_name": "coder-1234"
+}
+
+Prompt: "Fix https://github.com/org/repo/pull/567" →
+{
+	"display_name": "Fix org/repo PR #567",
+	"task_name": "repo-pr-567"
+}
+
 If a suitable name cannot be created, output exactly:
 {
 	"display_name": "Task Unnamed",