Make repository agent-ready

Add the metadata files AI agents (Claude Code, Cursor, Copilot, Codex) need to understand this codebase.

### Tasks

1. **`AGENTS.md`** (repo root) — canonical agent instructions: build, test, and lint commands; code conventions (formatting, type annotations, docstrings); architecture overview (package structure, Docker-based algorithm wrapping, enum-based algorithm selection); with links to CONTRIBUTING.md and docs/

2. **`docs/adr/`** — architecture decision records:
- `README.md` — what ADRs are, how to use them
- `0000-template.md` — copy-paste template for new ADRs
- Seed with at least 1 ADR covering a key decision (e.g., Docker wrapping)

3. **`docs/glossary.md`** — domain glossary: BraTS challenges, MRI modalities (t1c, t1n, t2f, t2w), atlases (SRI24, MNI152), task types, backends

4. **`.github/copilot-instructions.md`** — Copilot-specific instructions, linking to AGENTS.md

5. **Update `CONTRIBUTING.md`** — add reference to AGENTS.md and docs/adr/

### Acceptance Criteria
- [ ] `AGENTS.md` at repo root
- [ ] `docs/adr/` with README, `0000-template.md`, and >=1 seed ADR
- [ ] `docs/glossary.md`
- [ ] `.github/copilot-instructions.md`
- [ ] `CONTRIBUTING.md` updated with links to new files