fix: resolve relative links from source notes

[losnah-think]

/claim #274

Summary

• resolve explicit ./ and ../ wiki and Markdown links relative to the note containing the link
• preserve existing bare-name and vault-root path behavior while rejecting traversal outside the vault
• normalize Windows separators and escaped or URL-encoded spaces, and strip only the final .md extension so dotted stems survive
• generate relative file, heading, and block completions with the same lexical path rules

Validation

• cargo fmt --check
• cargo check --locked
• cargo test --locked — 86 passed
• cargo clippy --all-targets — no new warnings; existing main warnings remain
• git diff --check
• Neovim 0.12 headless A/B against the same temporary vault:
  • main: parent, sibling, and dotted relative definitions returned 0 targets; unresolved diagnostic remained; ../ completion absent
  • this branch: each definition returned 1 target; diagnostic cleared; ../available completion present
  • bare-name and vault-root links resolved in both versions

No TestFiles content was changed. This contribution was AI-assisted with OpenAI Codex and manually reviewed, including an independent diff review.