chore: Item 10 — migrate .scm metadata family → .a2ml (Track A, faithful) (#44)

Faithful lossless rename of META/ECOSYSTEM/AGENTIC/NEUROSYM/PLAYBOOK
`.scm`→`.a2ml` per the per-format specs (`standards/<fmt>-a2ml/spec/`):
`.a2ml` = same rich S-expression, `.scm` = deprecated legacy extension.
4/5 byte-identical; ECOSYSTEM Media-Type normalized
`vnd.ecosystem+scm`→`ecosystem+scm` per its IANA spec. STATE handled
separately as Track-B v2 thin-journal (#43, merged). All 5 validated
well-formed + losslessness-checked.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-authored-by: hyperpolymath <hyperpolymath@users.noreply.github.com>
Co-authored-by: Claude Opus 4.7 <noreply@anthropic.com>

Author: hyperpolymath

.machine_readable/AGENTIC.scm .machine_readable/AGENTIC.a2ml.machine_readable/AGENTIC.scm renamed to .machine_readable/AGENTIC.a2ml

@@ -1,6 +1,6 @@
 ;; SPDX-License-Identifier: AGPL-3.0-or-later
 ;; ECOSYSTEM.scm - Ecosystem position for modshells
-;; Media-Type: application/vnd.ecosystem+scm
+;; Media-Type: application/ecosystem+scm
 
 (ecosystem
   (version "1.0")