refactor(server): extract context.py into focused modules

Split the monolithic context.py (117 lines) into five purpose-specific
modules following single-responsibility principle:

- config.py: Server-level constants (DATA_DIR, SESSIONS_DIR, PORT,
  STALE_EVENT_AGE, _state_lock)
- agents.py: Agent-specific paths and caches (CLAUDE_PROJECTS_DIR,
  CODEX_SESSIONS_DIR, discovery caches)
- auth.py: Authentication token generation/validation for spawn endpoint
- spawn_config.py: Spawn feature configuration (PENDING_SPAWNS_DIR,
  rate limiting, projects watcher thread)
- zellij.py: Zellij binary resolution and session management constants

This refactoring improves:
- Code navigation: Find relevant constants by domain, not alphabetically
- Testing: Each module can be tested in isolation
- Import clarity: Mixins import only what they need
- Future maintenance: Changes to one domain don't risk breaking others

All mixins updated to import from new module locations. Tests updated
to use new import paths.

Includes PROPOSED_CODE_FILE_REORGANIZATION_PLAN.md documenting the
rationale and mapping from old to new locations.

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

tests/test_context.py

@@ -1,18 +1,18 @@
 import unittest
 from unittest.mock import patch
 
-from amc_server.context import _resolve_zellij_bin
+from amc_server.zellij import _resolve_zellij_bin
 
 
 class ContextTests(unittest.TestCase):
     def test_resolve_zellij_bin_prefers_which(self):
-        with patch("amc_server.context.shutil.which", return_value="/custom/bin/zellij"):
+        with patch("amc_server.zellij.shutil.which", return_value="/custom/bin/zellij"):
             self.assertEqual(_resolve_zellij_bin(), "/custom/bin/zellij")
 
     def test_resolve_zellij_bin_falls_back_to_default_name(self):
-        with patch("amc_server.context.shutil.which", return_value=None), patch(
-            "amc_server.context.Path.exists", return_value=False
-        ), patch("amc_server.context.Path.is_file", return_value=False):
+        with patch("amc_server.zellij.shutil.which", return_value=None), patch(
+            "amc_server.zellij.Path.exists", return_value=False
+        ), patch("amc_server.zellij.Path.is_file", return_value=False):
             self.assertEqual(_resolve_zellij_bin(), "zellij")