fix: continue file tree searches after link cycles

[CAOShurong]

Summary

• continue depth-first traversal after a path fails specifically with ErrLinkCycleDetected
• skip the same malformed entries during indexed file-tree searches instead of aborting all results
• warn with the affected path while preserving failures for every other error type
• add regressions proving files after a two-link cycle remain traversable and MIME-searchable

Fixes #206.

Validation

• go test ./pkg/filetree -count=1
• go test ./pkg/filetree -run 'TestDFS_WalkAll_SkipsLinkCycles|Test_searchContext_SearchByMIMEType_SkipsLinkCycles' -count=20
• go vet ./...
• standalone API probe using /usr/bin/xz <-> /usr/bin/xzcat: traversal and MIME search both return the normal files before and after the cycle with no error

The broader go test ./... run was also attempted on Windows. The changed pkg/filetree package passed; unrelated platform/integration suites retain existing Windows assumptions and require a Docker daemon, so I have not represented that command as passing.

AI assistance

OpenAI Codex assisted with issue screening, implementation, and test drafting. I reviewed the resulting diff and ran the validations listed above. The commit includes an Assisted-by trailer and DCO sign-off.