Remove redundant bisection orchestrator and stale docs

Drop bisect_agent.py: it was an unused standalone orchestrator that
duplicated the structured bisection/engine.py loop and git helpers. The
workflow and all wiring already use bisection_harness.py. Also remove the
superseded planning docs (architecture-overview, bootstrap, initial-poc),
keeping bisection-agent-overview.md as the single current reference.

Author: Neil4561