feat(retrieve): use tags metadata for cross-subtree retrieval

[13ernkastel]

Summary

• auto-extract and persist tags during resource vectorization, while also allowing user-supplied tags on resource ingestion and search requests
• thread tags through the resource/search services, SDK clients, and vector index filtering so callers can explicitly constrain retrieval by tag
• expand HierarchicalRetriever with bounded, down-weighted tag-based cross-subtree discovery before BFS traversal starts
• add focused retriever and router tests, and make the shared server test fixture use an isolated local config/AGFS setup

Related Issue

Fixes #1147

Why

Issue #1147 asks for tags to act as a lateral discovery signal across semantically distant subtrees. This keeps the existing hierarchical retrieval flow, but gives it a controlled way to discover related branches that the initial semantic top-K would otherwise miss.

Impact

• resources can store merged auto-extracted and user-provided tags
• find/search requests can explicitly scope retrieval by tags
• global semantic hits can seed additional related subtrees through shared tags with capped expansion and lower initial scores

Validation

• PYTHONPATH=/Users/lennonchia/Documents/Project/OpenViking /Users/lennonchia/Documents/Project/OpenViking/.venv/bin/python -m pytest -q tests/retrieve/test_hierarchical_retriever_target_dirs.py tests/retrieve/test_hierarchical_retriever_rerank.py tests/retrieve/test_hierarchical_retriever_tags.py tests/server/test_api_search.py::test_find_forwards_tags_to_service tests/server/test_api_resources.py::test_add_resource_forwards_tags_to_service

Notes

• I attempted the broader tests/server/test_api_search.py suite locally, but the current local wheel is missing the native VectorDB PersistStore symbol, so full end-to-end vector search verification is still environment-dependent here.

[github-actions]

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[MaojiaSheng]

我建议对 tag 的内容命名进行约束，例如强制声明 tag 来源或原因：
user:machine-learning;user:model-training;auto:pytorch