fix(mma): Change self.engine to self.engines dict for concurrent track support

- self.engine was a single ConductorEngine reference that got overwritten
  when multiple tracks ran concurrently, orphaning the first track's engine
- Now uses self.engines: Dict[str, ConductorEngine] keyed by track.id
- Updated _spawn_worker, kill_worker, pause_mma, resume_mma, approve_ticket,
  _load_active_tickets, and _update_ticket_depends_on to use engines.get(track_id)

Fixes concurrent MMA track execution bug where only one worker ever appeared.

.slop_cache/summary_cache.json

@@ -98,5 +98,13 @@
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+ },
+ "C:\\Users\\Ed\\AppData\\Local\\Temp\\pytest-of-Ed\\pytest-849\\test_auto_aggregate_skip0\\file1.txt": {
+  "hash": "d0b425e00e15a0d36b9b361f02bab63563aed6cb4665083905386c55d5b679fa",
+  "summary": "This document contains a single line of text, \"content1\". Its purpose and key takeaways are not discernible from the provided content.\n\n**Outline:**\n**TXT** \u2014 1 lines\npreview:\n```\ncontent1\n```"
+ },
+ "C:\\Users\\Ed\\AppData\\Local\\Temp\\pytest-of-Ed\\pytest-849\\test_force_full0\\other.txt": {
+  "hash": "04d61c0832f9cbc2a210334352425d2519890a0a5945da96ccc5bd9ff101c4d3",
+  "summary": "This document is a plain text file containing ten lines of content. The preview provided shows the first eight lines, indicating the file's straightforward, sequential nature.\n\n**Outline:**\n**TXT** \u2014 10 lines\npreview:\n```\nline1\nline2\nline3\nline4\nline5\nline6\nline7\nline8\n```"
  }
 }