Shorten wikilink block completion paths

[devin-ai-integration]

Summary

Fixes #421 by changing wikilink block completions to prefer filename-only block references when safe.

• Adds a shorten_block_ref_paths hook to distinguish wikilink completions from markdown link completions.
• Keeps markdown link block completions using the existing relative-path behavior.
• For wikilink block completions, inserts Filename#^blockid|display instead of folder/path/Filename#^blockid|display unless the short reference would be ambiguous.
• Falls back to the full vault-relative path when:
  • generating a new block ref for a file whose filename also appears elsewhere in the vault, or
  • completing an indexed block where another same-filename file has the same block id.
• Adds focused unit tests for duplicate filename/stem detection.

Review & Testing Checklist for Human

• Verify the ambiguity fallback matches expected Obsidian behavior for duplicate filenames and duplicate block IDs.
• Test in Neovim with [[ <query> block completion: unique filenames should insert Filename#^id|text, while duplicate filenames should still use the full path.
• Confirm markdown-link block completion behavior remains unchanged.

Notes

Local validation run:

• cargo fmt --check
• cargo clippy (passed with existing unrelated warnings)
• cargo test (passed with existing unrelated warning)

Link to Devin session: https://app.devin.ai/sessions/ed2e0a94bfdb4138995a0b7228ef4095
Requested by: @Feel-ix-343

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[devin-ai-integration]

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[devin-ai-integration]

✅ Devin Review: No Issues Found

Devin Review analyzed this PR and found no potential bugs to report.

View in Devin Review to see 3 additional findings.