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_discovery.py

@@ -27,7 +27,7 @@ class TestGetCodexPaneInfo(unittest.TestCase):
     def setUp(self):
         self.handler = DummyDiscoveryHandler()
         # Clear cache before each test
-        from amc_server.context import _codex_pane_cache
+        from amc_server.agents import _codex_pane_cache
         _codex_pane_cache["expires"] = 0
         _codex_pane_cache["pid_info"] = {}
         _codex_pane_cache["cwd_map"] = {}
@@ -80,7 +80,7 @@ class TestGetCodexPaneInfo(unittest.TestCase):
         self.assertEqual(pid_info["12345"]["zellij_session"], "myproject")
 
     def test_cache_used_when_fresh(self):
-        from amc_server.context import _codex_pane_cache
+        from amc_server.agents import _codex_pane_cache
         _codex_pane_cache["pid_info"] = {"cached": {"pane_id": "1", "zellij_session": "s"}}
         _codex_pane_cache["cwd_map"] = {"/cached/path": {"session": "s", "pane_id": "1"}}
         _codex_pane_cache["expires"] = time.time() + 100
@@ -203,7 +203,7 @@ class TestDiscoverActiveCodexSessions(unittest.TestCase):
     def setUp(self):
         self.handler = DummyDiscoveryHandler()
         # Clear caches
-        from amc_server.context import _codex_transcript_cache, _dismissed_codex_ids
+        from amc_server.agents import _codex_transcript_cache, _dismissed_codex_ids
         _codex_transcript_cache.clear()
         _dismissed_codex_ids.clear()
 
@@ -236,7 +236,7 @@ class TestDiscoverActiveCodexSessions(unittest.TestCase):
 
     def test_skips_dismissed_sessions(self):
         """Sessions in _dismissed_codex_ids should be skipped."""
-        from amc_server.context import _dismissed_codex_ids
+        from amc_server.agents import _dismissed_codex_ids
 
         with tempfile.TemporaryDirectory() as tmpdir:
             codex_dir = Path(tmpdir)