chore: remove dev artifacts; keep only published code/tests/docs

Remove agent/dev tooling and scratch files that aren't part of the published
project:
- .beads/ (issue tracker) and .clj-kondo/ (linter cache) — now gitignored
- AGENTS.md, CLAUDE.md, PLAN.md (agent/planning docs)
- root scratch scripts: fix-core.janet, preprocess.janet, and the loose
  test-*.janet probes (the real tests live under test/)
- clojure-features.clj — its features are already covered by
  test/integration/features-test.janet (dropped the optional smoke-test block)

The repo now tracks only: src/ (interpreter + clojure.core.async + stdlib),
test/ (spec/integration/unit), doc/grammar.ebnf, README, LICENSE, project.janet,
and the vendor/sci submodule. jpm test green.

.gitignore

@@ -9,3 +9,6 @@ test/_*.janet
 .dolt/
 *.db
 .beads-credential-key
+
+# Beads issue tracker (local dev tooling)
+.beads/