* fix: authorize guarded sends to active ConPTY agents
* Fix stale-handle rejection using controller-less status checks and contr
- Distinguish `terminal_handle_stale` (PTY rebind mid-check) from `terminal_exited`
so callers get an accurate reason instead of a generic `terminal_not_writable`.
- Move the PTY binding assertion to always run after the fresh foreground read,
even when no controller is present, closing a gap where a rebind during a
controller-less check went undetected.
- Call `confirmForegroundProcess` through its owning controller instance so the
method keeps its `this` receiver instead of being invoked unbound.
- Update reliability gates and design doc to reflect the new test coverage and
corrected error semantics.
* rm design doc
2802f0419d