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>

amc_server/auth.py

@@ -0,0 +1,18 @@
+"""Auth token generation and validation for spawn endpoint security."""
+
+import secrets
+
+# Auth token for spawn endpoint
+_auth_token: str = ''
+
+
+def generate_auth_token():
+    """Generate a one-time auth token for this server instance."""
+    global _auth_token
+    _auth_token = secrets.token_urlsafe(32)
+    return _auth_token
+
+
+def validate_auth_token(request_token: str) -> bool:
+    """Validate the Authorization header token."""
+    return request_token == f'Bearer {_auth_token}'