docs: overhaul Core runtime guides

[dajiaohuang]

Summary

• add bilingual task-oriented entry points for system consumers, data/transaction design, content packages, and retrieval
• clarify Core's boundary beneath MCP 2026 per-request identity and above authoritative storage, including the Web projection/cache rule and the absence of connection-bound tool/session authority in Core
• document compatible backup, migration, rollback, optional dependency, and test-data practices while fixing the stale hosted-service link

Validation

• uv sync --all-extras
• uv run --no-sync python -m pytest tests/test_auth_context.py tests/test_migrations.py tests/test_content_pack.py tests/test_state_documents.py tests/test_embeddings.py -q
• ruff check .
• git diff --check