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/server.py

@@ -1,7 +1,9 @@
 import os
 from http.server import ThreadingHTTPServer
 
-from amc_server.context import DATA_DIR, PORT, generate_auth_token, start_projects_watcher
+from amc_server.auth import generate_auth_token
+from amc_server.config import DATA_DIR, PORT
+from amc_server.spawn_config import start_projects_watcher
 from amc_server.handler import AMCHandler
 from amc_server.logging_utils import LOGGER, configure_logging, install_signal_handlers
 from amc_server.mixins.spawn import load_projects_cache