fix(search): use xargs -0 for null-delimited filelist to handle space…

[naman616]

…s in path (#670)

write_scoped_filelist wrote newline-delimited absolute paths; BSD xargs split each entry on whitespace, so a space in root_path (e.g. /Users/John Smith/...) caused grep to receive broken path fragments, silently producing zero matches.

Fix: write null-delimited paths (fputc '\0') and invoke xargs -0 so each path is passed as a single argument regardless of embedded spaces or metacharacters. Also add -H to the non-scoped grep -rn fallback so BSD grep always emits the filename prefix that collect_grep_matches requires.

Regression test: search_code_space_in_path_issue670 creates a project rooted at a directory containing a space, inserts an indexed node, and asserts that search_code returns a match — reproducing the exact reported failure scenario.

What does this PR do?

Checklist

• Every commit is signed off (git commit -s) — required, CI rejects
  unsigned commits (DCO, see CONTRIBUTING.md)
• Tests pass locally (make -f Makefile.cbm test)
• Lint passes (make -f Makefile.cbm lint-ci)
• New behavior is covered by a test (reproduce-first for bug fixes)

[DeusData]

Huge thanks for opening this PR and for the work you put into it.

The maintainer shop is currently full, so this may sit for a bit before it gets a proper review. We will come back to this as soon as possible with real feedback; I wanted to make sure it did not sit unacknowledged in the meantime.

[DeusData]

Thanks, this looks focused. Before merge, please rewrite the commit metadata to remove generated/co-author/session attribution. DCO otherwise appears to be present on the non-merge commit.