E2E tests for subagent feature

13 new E2E tests (KODE_E2E=true gated) in cmd/kode/subagent_e2e_test.go:

1. Real subprocess spawning (TestE2E_BinaryExists, TestE2E_SubagentRealProcess)
2. Stderr protocol (TestE2E_StdErrEmoji, TestE2E_QuietSuppressesStderr)
3. delegate_tasks tool with real binary (TestE2E_ToolSpawnsRealSubprocess,
   TestE2E_ToolAggregatesMultipleTasks, TestE2E_ToolReturnsJSONEmbedded)
4. Timeout wiring (TestE2E_ToolTimeout)
5. Concurrency limit (TestE2E_ToolConcurrencyLimit)
6. Edge cases (TestE2E_TaskFileViaTool, TestE2E_BinaryNotExecutable)
7. Stderr capture (TestE2E_StdErrCaptureInTool)

All tests use TestMain to build the binary once into a stable temp dir
(lifetime of the test suite, not per-test). All 12 packages pass with
race detector.

Author: jkyberneees