Fix byte counting, no-files-found output, and stdout/stderr mixing in grok_bridge.py

[Copilot]

Three correctness/UX bugs in the --write-files path of grok_bridge.py, surfaced in PR review.

Changes

• Bytes vs chars — replaced Path.write_text() (returns char count) with content.encode("utf-8", errors="replace") + write_bytes(encoded), recording len(encoded) so byte_count is actual UTF-8 bytes
• No annotated files found — when --write-files is set but no blocks match, previously dumped the full model response to stdout (defeating the compact-summary goal); now emits a short diagnostic to stderr:

  No annotated files found in model response to write to disk.
  Re-run without --write-files to see the full response.
  

• stdout/stderr consistency — Total: N bytes was going to stderr while the per-file list went to stdout; all write-summary output now goes to stdout
• --write-files + --output interop — --write-files was silently skipped when --output was provided; both now work independently (raw response written to --output, file extraction proceeds separately)

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