refactor(server): extract amc_server package from monolithic script

Split the 860+ line bin/amc-server into a modular Python package:

  amc_server/
    __init__.py         - Package marker
    context.py          - Shared constants (DATA_DIR, PORT, CLAUDE_PROJECTS_DIR, etc.)
    handler.py          - AMCHandler class using mixin composition
    logging_utils.py    - Structured logging setup with signal handlers
    server.py           - Main entry point (ThreadingHTTPServer)
    mixins/
      __init__.py       - Mixin package marker
      control.py        - Session control (dismiss, respond via Zellij)
      conversation.py   - Conversation history parsing (Claude JSONL format)
      discovery.py      - Session discovery (Codex pane inspection, Zellij cache)
      http.py           - HTTP response helpers (CORS, JSON, static files)
      parsing.py        - Session state parsing and aggregation
      state.py          - Session state endpoint logic

The monolithic bin/amc-server becomes a thin launcher that just imports
and calls main(). This separation enables:

- Easier testing of individual components
- Better IDE support (proper Python package structure)
- Cleaner separation of concerns (discovery vs parsing vs control)
- ThreadingHTTPServer instead of single-threaded (handles concurrent requests)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

amc_server/logging_utils.py

@@ -0,0 +1,31 @@
+import logging
+import signal
+import sys
+import threading
+
+LOGGER = logging.getLogger("amc.server")
+
+
+def configure_logging():
+    logging.basicConfig(
+        level=logging.INFO,
+        format="%(asctime)s %(levelname)s %(message)s",
+        stream=sys.stdout,
+        force=True,
+    )
+
+
+def install_signal_handlers(server):
+    """Log termination signals and shut down cleanly."""
+
+    def _handle(signum, _frame):
+        LOGGER.warning("Received signal %s, shutting down AMC server", signum)
+        # shutdown() blocks until serve_forever loop wakes; call from a helper thread.
+        threading.Thread(target=server.shutdown, daemon=True).start()
+
+    for sig in (signal.SIGTERM, signal.SIGINT, signal.SIGHUP):
+        try:
+            signal.signal(sig, _handle)
+        except (ValueError, OSError, RuntimeError):
+            # Some environments may restrict signal registration.
+            continue