fix: wire allowed_tools/disallowed_tools inputs through to Claude SDK

The disallowed tools env var read in run.ts was DISALLOWED_TOOLS_INPUT
(raw user input) instead of DISALLOWED_TOOLS (computed value including
WebSearch/WebFetch defaults). This meant default tool disallowance was
never enforced. Also removes orphaned ALLOWED_TOOLS env var export from
createPrompt() (tag mode passes allowed tools via claudeArgs instead),
extracts shared parseToolsInput utility to eliminate duplication, and
adds tests.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

src/utils/parse-tools-input.ts

@@ -0,0 +1,10 @@
+/**
+ * Parse a comma-separated tools string into a trimmed, non-empty array.
+ * Used for ALLOWED_TOOLS_INPUT and DISALLOWED_TOOLS_INPUT env vars.
+ */
+export function parseToolsInput(input: string | undefined): string[] {
+  return (input || "")
+    .split(",")
+    .map((t) => t.trim())
+    .filter(Boolean);
+}