docs(env): document sandcastle iteration env vars in .env.example

The three SANDCASTLE_*_ITERATIONS overrides landed inline in
decompose.mjs and dispatch.mjs (commit 26aa97f) but weren't
surfaced in .env.example. Adds them with the same tuning guidance
the inline comments carry, so users discover the knobs from the
canonical env reference instead of having to grep the code.

.env.example

@@ -73,3 +73,12 @@ CMS_URL=http://localhost:3001
 
 # Sandbox provider (default: docker; alternatives: podman, vercel, daytona)
 # SANDCASTLE_PROVIDER=docker
+
+# Agent iteration budgets. Sandcastle's `run()` cuts the agent off after N
+# iterations (one iteration = one tool-use + response round-trip). The
+# repo's defaults are tuned for typical work; bump if an agent gets cut
+# mid-commit (you'll see "Reached max iterations" in .sandcastle/logs/).
+#
+# SANDCASTLE_DECOMPOSE_ITERATIONS=10     # decompose: read PRD, write epic + stories, commit
+# SANDCASTLE_IMPLEMENTER_ITERATIONS=30   # implementer: full TDD slice (red test → green impl → gates → commit)
+# SANDCASTLE_REVIEWER_ITERATIONS=10      # reviewer: read diff + task, return decision