chore: Relicense from MIT to GNU AGPLv3

[Jam-Cai]

Summary

Relicenses Tabby from MIT to GNU Affero General Public License v3.0. Replaces LICENSE with the canonical text from gnu.org/licenses/agpl-3.0.txt and adds a brief License section to the README pointing at the file.

This is a relicense, not an addition — MIT permits sublicensing under more restrictive terms, so the relicense itself is legally clean. Practical effect: from this commit forward, the project is distributed under AGPLv3.

Validation

• LICENSE diff is the verbatim AGPLv3 text (660+ lines from gnu.org), no local edits.
• grep -rn -i "mit license\|Copyright (c) 2026 Tabby" --include="*.md" --include="*.swift" returns nothing outside the new LICENSE file — no stale references to update.
• No source-code or build-system changes — pure docs/license touch. Existing CI checks (Build, Lint, Tests) should pass without needing to re-run anything substantive.

Linked issues

None — this is a maintainer-level decision rather than tracking against a backlog item.

Risk / rollout notes

• Why AGPLv3 over MIT. The AGPL's network-use clause means any modified version made available to users over a network has to be source-available under the same terms. That matters if Tabby — currently fully on-device — ever evolves a network-served variant or a remote-model variant; AGPL keeps such derivatives source-available rather than becoming a closed fork.
• Existing contributions stay valid. Code already in the tree was written under MIT permission; nothing is being claimed retroactively. The MIT grant doesn't disappear from history. AGPLv3 governs the project going forward.
• Per-file SPDX identifiers are not added in this PR. If they're wanted, a follow-up sweep can drop // SPDX-License-Identifier: AGPL-3.0-or-later at the top of each .swift file.
• Dual-license alternative considered and not taken — single license is simpler and the typical AGPL pattern.
• Heads-up to other contributors is worth doing in a release note or pinned issue, since the license change shows up on the next release/clone.

[FuJacob]

thx for looking into this