Normalized the AI disclosure comment to one fixed line per file (#85)

The per-edit disclosure named the product and model, so every tool and every
model version appended another line rather than recognising the one already
there. Deduplication was by exact string, which a version bump defeats.

Every file now carries exactly one line, fixed text naming no product:

    Portions of this file were generated with AI assistance.

written with the comment character that file already uses. Precise attribution
stays on the commit, where the Assisted-by trailer is dated and attached to the
diff it describes. A header line cannot keep that record honest, because the
code it names gets rewritten and the line stays.

Comment-only, 15 files: every removed line was a disclosure line, every added
line is the fixed text, and no file is left with zero or more than one. Nested
repositories were excluded, so no submodule content or gitlink is touched.

Assisted-by: Claude Code (Opus 5) <noreply@anthropic.com>

test/regression/levelx_nor_flash_test_cache.c

@@ -9,7 +9,7 @@
 /* SPDX-License-Identifier: MIT                                            */
 /***************************************************************************/
 
-// Some portions generated by Codex (GPT-5).
+// Portions of this file were generated with AI assistance.
 
 /* Basic NOR flash tests...  */