docs: point agents to the shared test corpora (#90)

Real-file test oracles live in storytold/photocraft-corpus, explained in
craftrules standards/test-corpora.md. Never commit large binary fixtures
here; fetch them pinned and sha256-verified.

Co-authored-by: Claude Opus 5.5 <noreply@anthropic.com>

AGENTS.md

@@ -91,6 +91,7 @@ of it. Full standard: `../craftrules/standards/never-crash.md`
 - Shell gotcha: `mv`/`cp` are aliased interactive here — use `/bin/mv -f` / `/bin/cp -f`.
 - Parallel agents: separate git worktrees and `CARGO_TARGET_DIR=target/agent-<name>`; each agent uses its **own control port** (pick one in 18000–19999, never the default 7980) and its own scratch subfolder (`<scratch>/<agent-name>/`) — never `rm -rf` shared paths; delete your target dir when done (disk is shared); keep every `Cargo.toml` valid at all times (the `crates/*` glob means one broken manifest breaks everyone).
 - Test corpora: `cargo xtask corpus --download` into `corpus/` (gitignored, CC0 only). Never commit media.
+- Shared real-file test corpora (Photoshop-authored PSDs, etc.) live in [`storytold/photocraft-corpus`](https://github.com/storytold/photocraft-corpus), explained in [craftrules `standards/test-corpora.md`](https://github.com/storytold/craftrules/blob/main/standards/test-corpora.md). Never commit large binary fixtures to this repo; fetch them pinned by commit and sha256-verified, as PhotoCraft does with `cargo xtask corpus`.
 
 ## Testing & performance (do this often)
 - Unit/property tests next to the code; end-to-end tests drive real binaries (`crates/mcp/tests/e2e.rs`,