Restore deliberate AGPL-3.0-or-later licence (clobbered by estate MPL-normalise sweep #40)

[hyperpolymath]

Summary

the-nash-equilibrium is a deliberate AGPL-3.0-or-later exception to the estate
MPL-2.0 policy (co-developed game, same class as airborne-submarine-squadron). The
estate MPL-normalise sweep #40 (5e390d7) flattened its licence to MPL-2.0 without
honouring the exception. This is the second of exactly two genuine clobbers the
blast-radius analysis found (the other, airborne-submarine-squadron, is now restored —
see asq PR #57).

Unlike asq, this repo has no [CRITICAL] agpl-license invariant in its agent
instructions, so nothing stops a future sweep from re-clobbering it.

What to do (mirror asq PR #57 — proven recipe)

• Root LICENSE ← full GNU AGPL-3.0 text (not an SPDX stub — a stub reports
  NOASSERTION/"Other" on GitHub; only full text is detected as AGPL-3.0).
• LICENSES/ dir holding the full text of each licence in use:
  AGPL-3.0-or-later.txt (code) + CC-BY-SA-4.0.txt (docs).
• Flip code SPDX headers MPL-2.0 → AGPL-3.0-or-later; docs (.adoc/.md)
  stay CC-BY-SA-4.0. Target header lines, not a blind global sed.
• Flip machine-readable licence fields (META.a2ml, STATE.a2ml, etc.);
  leave correct exception-rationale prose that references MPL.
• Add the [CRITICAL] agpl-license invariant to AGENTS.md (+ mirrored agent
  instruction files) so the exception is self-documenting and sweep-proof.
• Add sonar-project.properties with sonar.exclusions/sonar.cpd.exclusions
  = LICENSE,LICENSES/** if SonarCloud is wired (verbatim licence text otherwise
  trips the duplication gate — see asq PR chore(ci): bump standards reusable workflow pins #58).

Acceptance criteria (proof, not diff)

• gh api repos/hyperpolymath/the-nash-equilibrium/license --jq '.license.spdx_id'
  returns AGPL-3.0 (currently MPL-2.0 or NOASSERTION).
• The governance / Licence consistency check passes (root classifier now recognises
  AGPL — standards PR #501, merged).
• Full test suite green from a space-free path (the _GAMES_GROUP _SET/ %20 in
  the path fake-fails extract tests otherwise).

References

• Root cause: estate sweep licence(#7): PMPL-1.0 -> PMPL-1.0-or-later (owner carve-out, SPDX-only) #40 (5e390d7).
• Restoration template: airborne-submarine-squadron PR chore(nix->guix): remove flake.nix (Guix-only) #57 (+ chore(ci): bump standards reusable workflow pins #58 for Sonar).
• Root classifier fix that makes AGPL pass governance: hyperpolymath/standards PR #501.