fix(mirror): send engine-only updates

[rissrice2105-agent]

Closes #108.

Summary

• track engine changes independently from buffered terminal output
• flush engine-only transitions both into an agent and back to the prompt
• avoid redundant updates when the engine value has not changed
• add a deterministic mirror test with injected credentials and HTTP transport

Validation

• node --test test/mirror.test.mjs (passed in three consecutive runs)
• full root suite: 247 passed, 5 skipped; the same 7 existing Windows failures require POSIX shell/signal behavior
• git diff --check