docs: add experiments/README.md — naming convention and run workflow

experiments/README.md

@@ -0,0 +1,32 @@
+# Experiments
+
+One directory per experiment run. Large outputs (embeddings, checkpoints, logs) are gitignored. Only metric summaries are committed to `results/summaries/`.
+
+## Naming convention
+
+```
+YYYYMMDD-HHMMSS-phase-N-short-description/
+  config.yaml       parameters used for this run (committed via results/summaries/)
+  metrics.json      final metric snapshot (committed via results/summaries/)
+  embeddings/       (gitignored — large)
+  checkpoints/      (gitignored — large)
+  logs/             (gitignored — large)
+```
+
+## Git tag convention
+
+Every run that produces reportable metrics gets a tag:
+
+```
+exp/YYYYMMDD-phase-N-description
+```
+
+Example: `exp/20260601-phase-0-mae-baseline`
+
+## Starting a run
+
+```bash
+task experiment:run -- --spec specs/phase-0-ssl-feasibility.md --tag my-run-description
+```
+
+The runner creates the directory, writes config, runs the experiment, and outputs metrics.json.