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/zellij.py

@@ -0,0 +1,34 @@
+"""Zellij integration: binary resolution, plugin path, session name, cache."""
+
+import shutil
+from pathlib import Path
+
+# Plugin path for zellij-send-keys
+ZELLIJ_PLUGIN = Path.home() / ".config" / "zellij" / "plugins" / "zellij-send-keys.wasm"
+
+
+def _resolve_zellij_bin():
+    """Resolve zellij binary even when PATH is minimal (eg launchctl)."""
+    from_path = shutil.which("zellij")
+    if from_path:
+        return from_path
+
+    common_paths = (
+        "/opt/homebrew/bin/zellij",  # Apple Silicon Homebrew
+        "/usr/local/bin/zellij",     # Intel Homebrew
+        "/usr/bin/zellij",
+    )
+    for candidate in common_paths:
+        p = Path(candidate)
+        if p.exists() and p.is_file():
+            return str(p)
+    return "zellij"  # Fallback for explicit error reporting by subprocess
+
+
+ZELLIJ_BIN = _resolve_zellij_bin()
+
+# Default Zellij session for spawning
+ZELLIJ_SESSION = 'infra'
+
+# Cache for Zellij session list (avoid calling zellij on every request)
+_zellij_cache = {"sessions": None, "expires": 0}