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_spawn.py

@@ -290,7 +290,7 @@ class TestRateLimiting(unittest.TestCase):
 
     def test_first_spawn_allowed(self):
         """First spawn for a project should not be rate-limited."""
-        from amc_server.context import _spawn_timestamps
+        from amc_server.spawn_config import _spawn_timestamps
         _spawn_timestamps.clear()
 
         handler = self._make_handler('fresh-project')
@@ -317,7 +317,7 @@ class TestRateLimiting(unittest.TestCase):
 
     def test_rapid_spawn_same_project_rejected(self):
         """Spawning the same project within cooldown returns 429."""
-        from amc_server.context import _spawn_timestamps
+        from amc_server.spawn_config import _spawn_timestamps
         _spawn_timestamps.clear()
         # Pretend we just spawned this project
         _spawn_timestamps['rapid-project'] = time.monotonic()
@@ -339,7 +339,7 @@ class TestRateLimiting(unittest.TestCase):
 
     def test_spawn_different_project_allowed(self):
         """Spawning a different project while one is on cooldown succeeds."""
-        from amc_server.context import _spawn_timestamps
+        from amc_server.spawn_config import _spawn_timestamps
         _spawn_timestamps.clear()
         _spawn_timestamps['project-a'] = time.monotonic()
 
@@ -360,7 +360,7 @@ class TestRateLimiting(unittest.TestCase):
 
     def test_spawn_after_cooldown_allowed(self):
         """Spawning the same project after cooldown expires succeeds."""
-        from amc_server.context import _spawn_timestamps, SPAWN_COOLDOWN_SEC
+        from amc_server.spawn_config import _spawn_timestamps, SPAWN_COOLDOWN_SEC
         _spawn_timestamps.clear()
         # Set timestamp far enough in the past
         _spawn_timestamps['cooled-project'] = time.monotonic() - SPAWN_COOLDOWN_SEC - 1