feat(ai): claude CLI subprocess driver (subscription auth, stream-json)

Implements runClaudeTurn() — spawns the claude CLI for a single companion
turn using subscription/OAuth auth (strips ANTHROPIC_API_KEY +
ANTHROPIC_AUTH_TOKEN from child env), streaming normalised events (delta,
tool, tool_result, result, error) via onEvent callback.

Includes hermetic test + fake-claude.js fixture that mimics real 2.1.159
stream-json output; zero network/CLI calls in the test suite.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

tests/fixtures/fake-claude.js

@@ -0,0 +1,66 @@
+#!/usr/bin/env node
+/**
+ * Fake claude CLI for hermetic tests.
+ *
+ * Mimics the stream-json output format of claude CLI 2.1.159:
+ * top-level events are either bare objects (system, assistant, result) or
+ * wrapped in {type:"stream_event", event:{...}}.
+ *
+ * Writes to stdout and exits 0.
+ */
+
+const lines = [
+  // system init (ignored by driver)
+  { type: 'system', subtype: 'init', session_id: 'fake-session-001', tools: [], cwd: '/tmp' },
+
+  // content_block_start: text block
+  { type: 'stream_event', event: { type: 'content_block_start', index: 0, content_block: { type: 'text', text: '' } } },
+
+  // text deltas — together they spell "Hello"
+  { type: 'stream_event', event: { type: 'content_block_delta', index: 0, delta: { type: 'text_delta', text: 'Hel' } } },
+  { type: 'stream_event', event: { type: 'content_block_delta', index: 0, delta: { type: 'text_delta', text: 'lo' } } },
+
+  // assistant snapshot (should be ignored by driver to avoid duplication)
+  { type: 'assistant', message: { role: 'assistant', content: [{ type: 'text', text: 'Hello' }] } },
+
+  // content_block_stop for text
+  { type: 'stream_event', event: { type: 'content_block_stop', index: 0 } },
+
+  // content_block_start: tool_use
+  { type: 'stream_event', event: { type: 'content_block_start', index: 1, content_block: { type: 'tool_use', id: 'toolu_fake01', name: 'propose_change', input: {} } } },
+
+  // tool input delta
+  { type: 'stream_event', event: { type: 'content_block_delta', index: 1, delta: { type: 'input_json_delta', partial_json: '{"file":"/tmp/x.js","content":"console.log(1)"}' } } },
+
+  // assistant snapshot for tool_use (ignored)
+  { type: 'assistant', message: { role: 'assistant', content: [{ type: 'tool_use', id: 'toolu_fake01', name: 'propose_change', input: { file: '/tmp/x.js', content: 'console.log(1)' } }] } },
+
+  // content_block_stop for tool
+  { type: 'stream_event', event: { type: 'content_block_stop', index: 1 } },
+
+  // tool_result event
+  { type: 'tool_result', tool_use_id: 'toolu_fake01', content: [{ type: 'text', text: 'change staged' }] },
+
+  // final result
+  {
+    type: 'result',
+    subtype: 'success',
+    is_error: false,
+    result: 'Hello',
+    stop_reason: 'end_turn',
+    session_id: 'fake-session-001',
+    total_cost_usd: 0.001234,
+    usage: {
+      input_tokens: 100,
+      output_tokens: 10,
+      cache_read_input_tokens: 0,
+      cache_creation_input_tokens: 0,
+    },
+  },
+];
+
+for (const line of lines) {
+  process.stdout.write(JSON.stringify(line) + '\n');
+}
+
+process.exit(0);