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/config.py

@@ -0,0 +1,20 @@
+"""Server-level constants: paths, port, timeouts, state lock."""
+
+import threading
+from pathlib import Path
+
+# Runtime data lives in XDG data dir
+DATA_DIR = Path.home() / ".local" / "share" / "amc"
+SESSIONS_DIR = DATA_DIR / "sessions"
+EVENTS_DIR = DATA_DIR / "events"
+
+# Source files live in project directory (relative to this module)
+PROJECT_DIR = Path(__file__).resolve().parent.parent
+DASHBOARD_DIR = PROJECT_DIR / "dashboard"
+
+PORT = 7400
+STALE_EVENT_AGE = 86400  # 24 hours in seconds
+STALE_STARTING_AGE = 3600  # 1 hour - sessions stuck in "starting" are orphans
+
+# Serialize state collection because it mutates session files/caches.
+_state_lock = threading.Lock()