docs: tighten AI Attribution section in AGENTS.md

Remove redundant intro sentence and 'For AI assistants:' label —
the whole document is addressed to AI assistants.

Assisted-by: Claude Code
Signed-off-by: Alex Bozarth <ajbozart@us.ibm.com>

Author: ajbozarth

AGENTS.md

@@ -97,9 +97,7 @@ Pre-commit runs: ruff, mypy, uv-lock, codespell
 
 ## 7. AI Attribution
 
-AI tools are welcome for development work in this project.
-
-**For AI assistants:** The project requires sign-offs — use `-s` when committing. Do not add a separate `Signed-off-by` line in the AI's own name. Add an `Assisted-by:` trailer to the commit footer by default:
+The project requires sign-offs — use `-s` when committing. Do not add a separate `Signed-off-by` line in the AI's own name. Add an `Assisted-by:` trailer to the commit footer by default:
 
 ```text
 Assisted-by: Claude Code