Duplicate `codeguard-0-safe-c-functions.md` in core and owasp rule sets

[thomas-bartlett]

Running the converter with --source core owasp — the documented way to
build with both rule sets — fails the duplicate-filename guard because two
files share a name:

• sources/rules/core/codeguard-0-safe-c-functions.md (328 lines)
• sources/rules/owasp/codeguard-0-safe-c-functions.md (140 lines)

The owasp copy is a strict subset of the core copy — same prose with bold
formatting added, minus ~190 lines of *_s() replacement patterns and
review checklists that only core has. Its frontmatter language list is
also wrong (lists javascript, ruby, typescript, yaml for a C
memory-safety rule).

CI didn't catch this: all three workflow invocations run the converter
with no --source arg, which defaults to core only, so the owasp set
is never loaded and the guard never sees both files together.

• Delete sources/rules/owasp/codeguard-0-safe-c-functions.md (or
  rename if there's a reason to keep a distinct OWASP variant)
• Make the duplicate guard in convert_to_ide_formats.py scan every
  dir under sources/rules/*/ regardless of --source, so a default
  run flags cross-set collisions
• Add a validate-rules.yml step that runs
  --source core owasp --output-dir /tmp/validate-all so CI also
  verifies the owasp rules convert cleanly

Both files are present on main.