Surface CLI task intake failures in chat task creation

[0xroylee]

What Changed

• Preserved CLI failure output when task creation or requirement intake fails in packages/server/src/tasks/task-chat-service.ts.
• Added a regression test in packages/server/tests/task-chat-service-failure.test.ts to verify stderr is surfaced instead of the generic fallback.

Why

• The chat submit flow was returning Task requirement intake failed even when the underlying CLI had a more useful error message.
• Surfacing the CLI stderr/stdout makes new-session chat failures actionable for operators and easier to diagnose.

How to Test

• Run the server package test suite and confirm the new regression test passes.
• Verify a failed task-intake command now reports the underlying CLI error text instead of the generic fallback.

Additional Notes

• bun run check still has unrelated existing formatting issues in packages/web, but server checks and tests pass.
• This change keeps the fallback message only for cases where the CLI provides no error output at all.