fix: AGENTIC licence line + a2ml-validate-action repin (standards#646, #669)

1. The AGENTIC.a2ml agent-constraint line "Never use AGPL license (...)"
   contradicts LICENCE-POLICY.adoc Rules 3 (co-developed), 4 (network
   services) and 5 (games), which MANDATE AGPL-3.0-or-later - and 144
   copies named the retired PMPL-1.0-or-later. Replaced with a pointer to
   the policy plus the A2 no-automated-licence-edits rule, hardcoding no
   licence so it cannot go stale again. Same wording as the template fix
   in rsr-template-repo#45; owner-ruled sweep (2026-08-27).

2. Any workflow pinning hyperpolymath/a2ml-validate-action at 59145c7d or
   e558e79200 is repinned to 6ac6416f. Those two SHAs never existed: the
   repo itself was only created 2026-08-28 and populated by subtree split
   from a2ml/actions/validate (286 files, history preserved). The old
   pins could never resolve and made lockfile generation impossible.

Direct push per owner ruling of 2026-08-28.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

Author: hyperpolymath

.machine_readable/6a2/AGENTIC.a2ml

@@ -21,7 +21,12 @@ can-create-files = true
 # - Never commit secrets or credentials
 # - Never use banned languages (TypeScript, Python, Go, etc.)
 # - Never place state files in repository root (must be in .machine_readable/)
-# - Never use AGPL license (use MPL-2.0)
+# - Never relicense an existing file, and never run an automated licence
+#   sweep (LICENCE-POLICY.adoc A2). New files get correct SPDX from birth.
+# - Never assume a licence. Read standards/LICENCE-POLICY.adoc: Rule 1
+#   defaults to MPL-2.0 (code) / CC-BY-SA-4.0 (prose), but Rule 3
+#   (co-developed), Rule 4 (network-deployed services) and Rule 5
+#   (games) are AGPL-3.0-or-later, and Rule 2 names the PMPL register.
 
 [maintenance-integrity]
 fail-closed = true