fix(mock_concurrent_mma): make epic branch a catch-all for non-empty prompts

The stress test (tests/test_mma_concurrent_tracks_stress_sim.py) uses
mma_epic_input='STRESS TEST: TRACK A AND TRACK B', which the mock's
epic branch did NOT match (it only matched 'PATH: Epic Initialization').
The stress prompt fell to the Default branch which returns text (not
JSON), and the production's orchestrator_pm.generate_tracks failed
to parse it, returning 0 tracks. The test polled for proposed_tracks
(60s timeout, never broke), clicked accept (no proposed_tracks to
process), then asserted tracks >= 2 and found 0.

Root cause: the mock's epic branch was a literal-substring check for
a single test-specific prompt. It was not robust to other test
prompts.

Fix: restructure routing so that sprint and worker are checked first
(more specific patterns), and ANY non-empty prompt that does not
match those patterns is treated as an epic request (returns 2
tracks). Empty prompts fall to the Default branch.

Verification:
- test_mma_concurrent_tracks_execution: still PASSES (uses
  'PATH: Epic Initialization' which matches the new catch-all since
  it doesn't contain sprint or worker patterns)
- test_mma_concurrent_tracks_stress_sim: now PASSES (uses
  'STRESS TEST: TRACK A AND TRACK B' which matches the new catch-all)
- 3 consecutive PASS runs of both tests (13.94s, 14.81s, 14.13s)

This is 'adjust the tests instead' per user directive - the mock is
a test artifact, not production. The production's generate_tracks
correctly returns [] for unparseable responses; the test mock should
be robust enough to return valid JSON for any epic-like prompt.

tests/mock_concurrent_mma.py

@@ -47,26 +47,7 @@ def main() -> None:
 
  call_n = _next_call_count()
 
- # 1. Epic Initialization
- if 'PATH: Epic Initialization' in prompt:
-  mock_response = [
-   {"id": "track-a", "goal": "Track A Goal", "title": "Track A"},
-   {"id": "track-b", "goal": "Track B Goal", "title": "Track B"}
-  ]
-  print(json.dumps({
-   "type": "message",
-   "role": "assistant",
-   "content": json.dumps(mock_response)
-  }), flush=True)
-  print(json.dumps({
-   "type": "result",
-   "status": "success",
-   "stats": {"total_tokens": 100, "input_tokens": 50, "output_tokens": 50},
-   "session_id": "mock-epic"
-  }), flush=True)
-  return
-
- # 2. Sprint Planning (different tickets for different tracks)
+ # 1. Sprint Planning (different tickets for different tracks)
  # Route on prompt content (the production passes the track_brief which
  # contains "Track A" or "Track B"). The prior session_id-based routing was
  # fragile because:
@@ -75,6 +56,7 @@ def main() -> None:
  #   2. session_id="mock-sprint-A" means "this is a follow-up call after
  #      the 1st sprint returned mock-sprint-A", so the response should be
  #      sprint-B (2nd track), not sprint-A.
+ # CHECK BEFORE epic so sprint takes priority over the catch-all epic branch.
  if 'generate the implementation tickets' in prompt:
   if "Track A" in prompt: track_label = "A"
   elif "Track B" in prompt: track_label = "B"
@@ -83,7 +65,8 @@ def main() -> None:
   _emit_sprint_ticket(track_label)
   return
 
- # 3. Worker Execution
+ # 2. Worker Execution
+ # CHECK BEFORE epic so worker takes priority over the catch-all epic branch.
  if 'You are assigned to Ticket' in prompt or session_id.startswith("mock-worker-"):
   import re
   match = re.search(r'Ticket (ticket-[A-Ba-b]-1)', prompt, re.IGNORECASE)
@@ -107,6 +90,31 @@ def main() -> None:
   }), flush=True)
   return
 
+ # 3. Epic Initialization (catch-all for any non-empty prompt that
+ # does not match the sprint or worker patterns above). This makes the
+ # mock robust to test-specific epic prompts (e.g. 'STRESS TEST: TRACK A
+ # AND TRACK B' used by test_mma_concurrent_tracks_stress_sim). The
+ # prior version only matched 'PATH: Epic Initialization', so other
+ # prompts fell to the Default branch and the production failed to parse
+ # the response as JSON, returning 0 tracks.
+ if prompt.strip():
+  mock_response = [
+   {"id": "track-a", "goal": "Track A Goal", "title": "Track A"},
+   {"id": "track-b", "goal": "Track B Goal", "title": "Track B"}
+  ]
+  print(json.dumps({
+   "type": "message",
+   "role": "assistant",
+   "content": json.dumps(mock_response)
+  }), flush=True)
+  print(json.dumps({
+   "type": "result",
+   "status": "success",
+   "stats": {"total_tokens": 100, "input_tokens": 50, "output_tokens": 50},
+   "session_id": "mock-epic"
+  }), flush=True)
+  return
+
  # Default
  print(json.dumps({
   "type": "message",