v0.7.6

Changed

• The OS is now abstracted behind a registry of seams, so a non-tmux/native-Windows port is new
  files plus one registration line each — no core edits. The terminal multiplexer is selected
  through a registry (register_multiplexer, by sys.platform with a BMAD_AUTO_MUX_BACKEND
  override) rather than hardcoded in get_multiplexer(), and the tmux backend split into a reusable
  BaseTmuxBackend extension point — every tmux invocation funnels through one overridable _run()
  primitive — with TmuxMultiplexer as a thin POSIX leaf, so a tmux-family backend (an eventual
  "psmux") overrides only the spawn primitive and the few divergent methods.
• Process-lifecycle primitives moved behind a ProcessHost seam. Politely-stop / force-kill /
  liveness / PID-reuse-identity now route through get_process_host() (registered like the
  multiplexer, with a BMAD_AUTO_PROCESS_HOST override); a WindowsProcessHost ships ready to
  register. Hook registration no longer hardcodes python3 — it takes the interpreter prefix from
  ProcessHost.hook_interpreter() (POSIX python3; Windows uv run --no-project python), and
  bmad-auto validate runs a platform preflight that reports the selected backend's readiness and
  names the process host. Behavior on Linux/macOS/WSL is unchanged.
• The bundled Unity plugin's teardown delegates pid lifecycle to ProcessHost and its helper
  scripts run under the orchestrator's own interpreter. Plugin helper scripts are spawned via
  sys.executable (not a PATH-resolved python3), so a bundled script may import automator seams;
  unity_teardown.py now reaps leaked Editor/MCP processes through get_process_host() instead of
  re-implementing kill/liveness, gaining Windows behavior for free.

Added

• A consolidated porting guide maps the four OS seams (terminal
  multiplexer, process lifecycle, hook interpreter, validate preflight), their registries and
  test-override env vars, and exactly what a native-Windows port costs end to end. The
  adapter-/plugin-authoring guides, ROADMAP, README, and FEATURES are updated to the post-registry
  world.