fix(cli): preserve SynthID uncertainty in no-visible-mark message

The 'no signal' branch of the visible no-mark path claimed 'No AI provenance
signal found either', which reads as 'the image is clean'. A missing metadata
proxy is not proof an invisible pixel watermark (SynthID) is absent: it cannot
be detected once metadata is gone and may have been stripped upstream. The
message now preserves that uncertainty and routes to both 'all' (regenerate
pixels) and 'erase'. Regression-guarded by the SynthID/all asserts in
test_cli.py. CLAUDE.md visible-command note updated to match.

Also adds a 'Scope and non-goals' section (CLAUDE.md + README): removing
AI-provenance marks on the user's own content is in scope; stripping
stock/paid-content watermarks (Shutterstock/Getty/iStock, classifieds) is out
of scope by principle, not by difficulty.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

src/remove_ai_watermarks/cli.py

@@ -349,8 +349,12 @@ def _no_visible_mark_exit(source: Path) -> NoReturn:
         )
     else:
         console.print(
-            "  No AI provenance signal found either. If there is a logo or object to remove,\n"
-            "  target it directly with the region eraser:\n"
+            "  No visible mark and no readable AI provenance signal. This does not prove\n"
+            "  the image is clean: an invisible pixel watermark such as SynthID cannot be\n"
+            "  detected here once the metadata proxy is absent (it may have been stripped\n"
+            "  earlier). If the image is AI-generated, regenerate the pixels with:\n"
+            f"    remove-ai-watermarks all {source.name}\n"
+            "  If instead there is a logo or object to remove, target it with the region eraser:\n"
             f"    remove-ai-watermarks erase {source.name} --region x,y,w,h"
         )
     raise SystemExit(EXIT_NO_VISIBLE_MARK)