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/handler.py

@@ -0,0 +1,27 @@
+from http.server import BaseHTTPRequestHandler
+
+from amc_server.mixins.conversation import ConversationMixin
+from amc_server.mixins.control import SessionControlMixin
+from amc_server.mixins.discovery import SessionDiscoveryMixin
+from amc_server.mixins.http import HttpMixin
+from amc_server.mixins.parsing import SessionParsingMixin
+from amc_server.mixins.state import StateMixin
+
+
+class AMCHandler(
+    HttpMixin,
+    StateMixin,
+    ConversationMixin,
+    SessionControlMixin,
+    SessionDiscoveryMixin,
+    SessionParsingMixin,
+    BaseHTTPRequestHandler,
+):
+    """HTTP handler composed from focused mixins."""
+
+    def handle(self):
+        """Ignore expected disconnect noise from short-lived HTTP/SSE clients."""
+        try:
+            super().handle()
+        except (ConnectionResetError, BrokenPipeError):
+            return