chore(a2ml): add MPL-2.0 SPDX headers to clear validator warnings

The A2ML validator warned "Missing SPDX-License-Identifier in first 10
lines" on several manifests. Add the SPDX header (within the first 10
lines) to the non-canonical ones:
- .github/0.1-AI-MANIFEST.a2ml
- .machine_readable/ai/AI.a2ml
- examples/0.1-AI-MANIFEST.a2ml
- verification/tests/0.2-AI-MANIFEST.a2ml

The two 6a2-canonical manifests that also warn
(.machine_readable/6a2/0-AI-MANIFEST.a2ml and
.machine_readable/6a2/anchor/0-AI-MANIFEST.a2ml) are deliberately left
untouched — they are warnings (not gate-failing errors) and the 6a2
canonical files are out of scope for this cleanup pass.

https://claude.ai/code/session_013wg3Mtq2QFhYi4XVw1Z6z7

Author: claude

.github/0.1-AI-MANIFEST.a2ml

@@ -1 +1,2 @@
+# SPDX-License-Identifier: MPL-2.0
 # AI Manifest - Level 1: .github

.machine_readable/ai/AI.a2ml

@@ -1,3 +1,4 @@
+# SPDX-License-Identifier: MPL-2.0
 
 # AI Assistant Instructions
 

examples/0.1-AI-MANIFEST.a2ml

@@ -1 +1,2 @@
+# SPDX-License-Identifier: MPL-2.0
 # AI Manifest - Level 1: examples

verification/tests/0.2-AI-MANIFEST.a2ml

@@ -1 +1,2 @@
+# SPDX-License-Identifier: MPL-2.0
 # AI Manifest - Level 1: tests