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/mixins/http.py

@@ -1,8 +1,8 @@
 import json
 import urllib.parse
 
-import amc_server.context as ctx
-from amc_server.context import DASHBOARD_DIR
+import amc_server.auth as auth
+from amc_server.config import DASHBOARD_DIR
 from amc_server.logging_utils import LOGGER
 
 
@@ -148,10 +148,10 @@ class HttpMixin:
             content_type = content_types.get(ext, "application/octet-stream")
 
             # Inject auth token into index.html for spawn endpoint security
-            if file_path == "index.html" and ctx._auth_token:
+            if file_path == "index.html" and auth._auth_token:
                 content = content.replace(
                     b"<!-- AMC_AUTH_TOKEN -->",
-                    f'<script>window.AMC_AUTH_TOKEN = "{ctx._auth_token}";</script>'.encode(),
+                    f'<script>window.AMC_AUTH_TOKEN = "{auth._auth_token}";</script>'.encode(),
                 )
 
             # No caching during development