Harden create-pr-from-issue workflow script construction

[Copilot]

The create-pr-from-issue Actions job failed at runtime with SyntaxError: Unexpected end of input, preventing Copilot task creation from issue events.
Root cause was malformed JavaScript in the workflow’s actions/github-script step (unterminated multiline template literal).

• Workflow script reliability

  • Replaced fragile multiline template-literal assembly of problem_statement with deterministic line-array construction and join("\n").
  • Preserves identical payload content while eliminating parser-failure risk from accidental delimiter imbalance.

• Action runtime alignment

  • Updated actions/github-script usage to v9 in the workflow to align with current runner runtime expectations.

• Behavioral intent preserved

  • Workflow still:
    • triggers on issue open,
    • creates a Copilot task with rich issue context/instructions,
    • posts a follow-up issue comment.

const problemStatement = [
  "A new GitHub issue was opened and should be analyzed and implemented.",
  "",
  `Repository: ${owner}/${repo}`,
  `Issue: #${issue.number}`,
  ...
].join("\n");