fix: Escape special regex chars in buffer lookup

[seflue]

Buffer lookup failed for filenames containing [], (), . and other regex metacharacters.

• Add utils.get_buffer_by_filename() with proper Vim regex escaping
• Replace duplicate buffer lookup patterns across files

Summary

This PR fixes buffer lookup failures when filenames contain regex special characters like [], (), ., etc. The issue occurred because vim.fn.bufnr() patterns weren't properly escaped for Vim regex syntax, causing buffer operations (todo state, links, refiling) to fail.

Related Issues

Closes #1021

Changes

• Add utils.get_buffer_by_filename() utility function with proper Vim regex escaping using vim.fn.escape()
• Replace 6 duplicate buffer lookup patterns across 4 files (files/file.lua, api/init.lua, api/headline.lua,
  api/file.lua)
• Add comprehensive test coverage for special character scenarios in tests/plenary/utils_spec.lua

Checklist

I confirm that I have:

• Followed the
  Conventional Commits
  specification (e.g., feat: add new feature, fix: correct bug,
  docs: update documentation).
• My PR title also follows the conventional commits specification.
• Updated relevant documentation, if necessary.
• Thoroughly tested my changes.
• Added tests (if applicable) and verified existing tests pass with
  make test.
• Checked for breaking changes and documented them, if any.

[kristijanhusak]

Thanks!