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/spawn.py

@@ -5,13 +5,52 @@ import subprocess
 import time
 import uuid
 
-from amc_server.context import (
-    PROJECTS_DIR, SESSIONS_DIR, ZELLIJ_BIN, ZELLIJ_SESSION,
+from amc_server.auth import validate_auth_token
+from amc_server.config import SESSIONS_DIR
+from amc_server.spawn_config import (
+    PENDING_SPAWNS_DIR, PENDING_SPAWN_TTL,
+    PROJECTS_DIR,
     _spawn_lock, _spawn_timestamps, SPAWN_COOLDOWN_SEC,
-    validate_auth_token,
 )
+from amc_server.zellij import ZELLIJ_BIN, ZELLIJ_SESSION
 from amc_server.logging_utils import LOGGER
 
+
+def _write_pending_spawn(spawn_id, project_path, agent_type):
+    """Write a pending spawn record for later correlation by discovery.
+
+    This enables Codex session correlation since env vars don't propagate
+    through Zellij's pane spawn mechanism.
+    """
+    PENDING_SPAWNS_DIR.mkdir(parents=True, exist_ok=True)
+    pending_file = PENDING_SPAWNS_DIR / f'{spawn_id}.json'
+    data = {
+        'spawn_id': spawn_id,
+        'project_path': str(project_path),
+        'agent_type': agent_type,
+        'timestamp': time.time(),
+    }
+    try:
+        pending_file.write_text(json.dumps(data))
+    except OSError:
+        LOGGER.warning('Failed to write pending spawn file for %s', spawn_id)
+
+
+def _cleanup_stale_pending_spawns():
+    """Remove pending spawn files older than PENDING_SPAWN_TTL."""
+    if not PENDING_SPAWNS_DIR.exists():
+        return
+    now = time.time()
+    try:
+        for f in PENDING_SPAWNS_DIR.glob('*.json'):
+            try:
+                if now - f.stat().st_mtime > PENDING_SPAWN_TTL:
+                    f.unlink()
+            except OSError:
+                continue
+    except OSError:
+        pass
+
 # Agent commands (AC-8, AC-9: full autonomous permissions)
 AGENT_COMMANDS = {
     'claude': ['claude', '--dangerously-skip-permissions'],
@@ -215,6 +254,16 @@ class SpawnMixin:
 
     def _spawn_agent_in_project_tab(self, project, project_path, agent_type, spawn_id):
         """Spawn an agent in a project-named Zellij tab."""
+        # Clean up stale pending spawns opportunistically
+        _cleanup_stale_pending_spawns()
+
+        # For Codex, write pending spawn record before launching.
+        # Zellij doesn't propagate env vars to pane commands, so discovery
+        # will match the session to this record by CWD + timestamp.
+        # (Claude doesn't need this - amc-hook writes spawn_id directly)
+        if agent_type == 'codex':
+            _write_pending_spawn(spawn_id, project_path, agent_type)
+
         # Check session exists
         if not self._check_zellij_session_exists():
             return {