chore(license): migrate to AGPL-3.0-or-later (#64)

## Summary
Migrates this repo's license declaration from `MPL-2.0` to
`AGPL-3.0-or-later`.

**Rationale:** Joshua co-dev.

## Changes
- LICENSE file replaced with canonical `AGPL-3.0-or-later` text
(including SPDX header).
- Package metadata updated where present (Cargo.toml, package.json,
deno.json, mix.exs, pyproject.toml).

Part of estate-wide metadata cleanup 2026-05-26.

Author: hyperpolymath