feat(fs): 新增 attrs 元信息接口

[qin-ctx]

Description

新增 GET /api/v1/fs/attrs，用于读取文件或目录的逻辑扩展属性。当前返回：

• attrs.tags：显式检索标签，对外统一命名为 tags
• attrs.memory：memory 文件里的 MEMORY_FIELDS 元信息，并去掉正文 content

同时补齐 Python SDK、Go SDK、Rust CLI 和中英文文档。

Related Issue

N/A

Type of Change

• Bug fix (non-breaking change that fixes an issue)
• New feature (non-breaking change that adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Documentation update
• Refactoring (no functional changes)
• Performance improvement
• Test update

Changes Made

• 新增 /api/v1/fs/attrs，返回 uri、context_type 和 attrs，其中 attrs.tags 来自向量记录里的 search_tags，attrs.memory 来自 memory 文件的 MEMORY_FIELDS。
• 新增 CLI：openviking attrs get <uri> [key] 和 openviking attrs set-tags ...；旧的顶层 openviking set-tags 保留兼容但隐藏在 help 外。
• 新增 SDK 方法和文档示例，覆盖 Python、Go、HTTP、CLI，以及 resource/memory 两种响应。

Testing

• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes
• I have tested this on the following platforms:
  • Linux
  • macOS
  • Windows

运行过：

pytest tests/server/test_filesystem_router.py -q
pytest sdk/python/tests/test_async_client_behaviors.py -q
cargo check -p ov_cli
GOCACHE=/tmp/openviking-go-build go test ./...

cargo fmt -p ov_cli -- --check 未通过，失败点是仓库里已有的 ov_cli 无关格式化差异；本 PR 没有全量 rustfmt，避免扩大 diff。

Checklist

• My code follows the project's coding style
• I have performed a self-review of my code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• My changes generate no new warnings
• Any dependent changes have been merged and published

Screenshots (if applicable)

N/A

Additional Notes

示例：

openviking attrs get viking://resources/docs/api.md
openviking attrs get viking://resources/docs/api.md tags
openviking attrs get viking://user/alice/memories/experiences/foo.md memory.resource_refs
openviking attrs set-tags viking://resources/docs/api.md --tags team=search,env=prod
openviking attrs set-tags viking://resources/docs --tags team=search --mode append --recursive

返回示例：

{
  "uri": "viking://user/alice/memories/experiences/foo.md",
  "context_type": "memory",
  "attrs": {
    "memory": {
      "memory_type": "experiences",
      "name": "foo",
      "tags": ["ui"],
      "resource_refs": ["viking://resources/docs/api.md"]
    },
    "tags": ["team=search"]
  }
}