Rename remote app to observer across codebase
Directory renames: apps/remote -> apps/observer, observe/remote_cli.py -> observe/observer_cli.py, observe/remote_client.py -> observe/observer_client.py
All API routes, blueprints, CLI commands, utility functions, config keys, env vars, callosum event fields, JSONL metadata keys, tests, and docs updated.
Migration maint task added at apps/observer/maint/000_migrate_remote_to_observer.py.
No behavioral changes - pure rename with clean break (no backward compat aliases).