Auto-load prior DSG into Neo4j on chatdsg startup

[harelb]

Fixes MIT-SPARK/Awesome-DCIST-T4#337.

Problem

Two separate things were wrong:

1. The load path was broken. chatdsg.py called load_dsg_to_db() with the old five-argument signature (object_labelspace, room_labelspace, uri, creds, graph), but heracles.utils.load_dsg_to_db() now takes (neo4j_uri, neo4j_creds, scene_graph) and reads labelspaces out of the graph metadata. Passing --scene-graph raised TypeError.
2. Nothing ever triggered a load. The launch components in dcist_launch_system invoke ./chatdsg.py with no arguments, and the load was gated on if args.scene_graph:, so the prior scene graph was never loaded into Neo4j.

Changes

• --scene-graph now defaults to $ADT4_PRIOR_MAP/hydra/backend/dsg.json — the layout the launch system already documents and forwards into the chatdsg pane.
• Fixed the load_dsg_to_db() call and removed --object-labelspace / --room-labelspace. Labelspaces come from the graph. extract_labelspaces_from_dsg() is used to warn when a graph carries none, instead of silently loading unlabeled objects and rooms.
• Added --no-dsg-load. This is the one addition beyond the issue: loading calls initialize_db(), which clears the database, so auto-loading means every startup wipes Neo4j. Worth a deliberate look — there is an existing feature/chatdsg_optional_dsg_reset branch suggesting the same need.
• Credentials fall back to ADT4_NEO4J_* when HERACLES_NEO4J_* is unset, so the example works when run directly rather than under tmuxp.
• logging.basicConfig() so the startup messages actually print before the Textual app takes the terminal.

No config regeneration is required: dcist_launch_system/scripts/generate_configs.sh produces a zero diff, and both tmux configs that launch chatdsg already forward ADT4_PRIOR_MAP and map HERACLES_NEO4J_* from ADT4_NEO4J_*.

Testing

Run against a live Neo4j with example_dsg.json staged as a prior map.

• Invoked chatdsg.py with no CLI arguments, exactly as the tmux components do: loaded 65 objects / 96 2D places / 5 rooms.
• Confirmed by Cypher query that labels came from the graph's embedded labelspace, not the YAML files — objects resolved to seating, storage, sign, decor; rooms to hallway, lounge.
• --no-dsg-load skips the load; unset $ADT4_PRIOR_MAP warns instead of failing.
• Guard paths verified: missing DSG file and missing credentials both skip without contacting the database.
• pre-commit clean (ruff, ruff-format, whitespace).

Note that --scene-graph keeps nargs="?", so a bare --scene-graph with no value still falls through to the $ADT4_PRIOR_MAP default.