Handle relative paths for codecompanion tools

[Davidyz]

Closes #260

Since other tools in codecompanion.nvim are designed to avoid IO on files outside of cwd, this PR differentiate paths inside of cwd and outside of cwd by using relative paths for the former, and absolute paths for the later. Instructions are included in the query and files_ls tool schemas so that the model knows that they can't use the other tools on files represented by absolute paths.

Until the upstream rule of "no IO outside of cwd" is changed, this extension will stick to this rule when formatting the output.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 99.77%. Comparing base (55b4d03) to head (e5e760b).

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #267   +/-   ##
=======================================
  Coverage   99.77%   99.77%           
=======================================
  Files          24       24           
  Lines        1775     1775           
=======================================
  Hits         1771     1771           
  Misses          4        4           

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