Skip generated RepoBrain artifacts during scans

[Lling0000]

Summary

• exclude .repobrain and artifacts from the shared scan/search skip set
• prevent full_scan from counting generated artifact files as project source
• prevent structure.md generation from including previous RepoBrain outputs

Why

RepoBrain writes its own knowledge files under .repobrain and stores generated plans/reports under artifacts. Those directories were only skipped by module detection, not by the shared scanner/structure path. As a result a second refresh could include .repobrain/structure.md and artifacts/*.md in the generated project structure and scan metadata, polluting future refresh and ask context with generated outputs instead of source files.

Validation

• python -m pytest engine/tests/test_hub_scanner.py -q
• python -m pytest engine/tests/test_ask_tools.py -q
• python -m pytest engine/tests/test_hub_semantic_graph.py -q
• python -m pytest engine/tests/test_plugin_packaging.py -q
• manual minimal reproduction confirms scan metadata and structure output only include src/app.py