chore: migrate .scm metadata family → .a2ml (Track A, Estate Item 10)

Faithful, lossless migration of the remaining 5 machine-readable
metadata files to the `.a2ml` extension, per the per-format specs in
standards/<fmt>-a2ml/spec/ (FORMAT-SPEC + ABNF + JSON schema): the
`.a2ml` form IS the same rich S-expression content; `.scm` is the
deprecated legacy extension (deprecated estate-wide 2026-04-12).

- META/AGENTIC/NEUROSYM/PLAYBOOK: byte-identical content, pure git mv.
- ECOSYSTEM: Media-Type header normalized from the deprecated
  `application/vnd.ecosystem+scm` to the canonical
  `application/ecosystem+scm` (per ecosystem-a2ml IANA spec); body
  otherwise byte-identical.

STATE was migrated separately to the Track-B v2 thin-journal (#43,
merged) — it is the one format with an intentional reductive redesign.

Validated: all 5 well-formed S-expressions, content losslessness
verified against the pre-rename blobs.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

Author: hyperpolymath