chore: add a root LICENSE, and relicense genkit_flutter_gemma to MIT

[DenisovAV]

Two licensing gaps, found while checking why the site footer says MIT.

No LICENSE at the repository root

Licence files exist only inside each packages/*/. GitHub therefore reports the repository's licence as null:

$ gh api repos/DenisovAV/flutter_gemma --jq '.license.spdx_id'
null

So the repo page shows no licence badge, and automated scanners — including the corporate ones used to vet dependencies — read the project as "no licence stated". For a package people pull into production that is the signal that makes a legal review say no.

pub.dev was never affected: it reads the file from inside each published archive, and every package has one.

Added a root LICENSE with the same MIT text the packages use.

genkit_flutter_gemma was BSD-3-Clause

Eleven packages are MIT; this one was not. Verified against the published archives, not the pub.dev page:

genkit_flutter_gemma  v0.6.0   BSD 3-Clause
genkit_hybrid         v0.2.1   MIT License
flutter_gemma         v1.8.0   MIT

It arrived that way with the import in #331 — genkit_hybrid came in the same commit and is MIT — so this reads as drift rather than a decision. Its copyright line names Flutter Berlin / Sasha Denisov, not Google, so it is not a leftover from a Genkit template either.

Relicensed to MIT, keeping its own copyright line. MIT is strictly more permissive than BSD-3-Clause (it drops the no-endorsement clause), so nobody already using the package loses a right. Versions already on pub.dev stay BSD-3-Clause; MIT applies from 0.6.1, which this PR bumps — LICENSE ships inside the archive, so without a release the change reaches nobody.

Pins deliberately untouched

^0.6.0 already admits 0.6.1, and the codelabs under codelabs/ carry real pubspec.yaml files. Naming ^0.6.1 there before it is published would break resolution for anyone following the tutorial — the same trap as a satellite floor naming an unpublished core.

Noted, not changed

• Copyright years differ across packages (2024 / 2025 / 2026) — that is the year each was first published, so rewriting them would be wrong.
• flutter_gemma_onnx and genkit_hybrid differ from the canonical text only by a trailing newline and an MIT License header line.