docs(reborn): converge subagent harness plan (7 thermo-nuclear rounds)

Looped thermo-nuclear review over the PR0+PR1 implementation plan until the
reviewer flagged nothing above LOW. Across 7 rounds, fixed every code-grounded
correctness/compile issue:

- Blocking reuses TurnStatus::BlockedDependentRun + a real await-edge GateRef
  (gate:subagent-await-<child>, hyphen for LoopGateRef validity) instead of a
  new variant (design §3 reversed; user-approved CX decision — subagent waits
  show as ordinary dependent-run waits).
- Edge lifecycle settle -> resume -> drain (drain = parent consumed); resolver
  collects Settled edges incl. settled-before-crash; boot recovery resumes via
  the durable edge (closes the restart-delivery hole).
- edge.child_scope captured verbatim from finish_spawn's submit scope so the
  resolver looks the child up under the scope it was stored with.
- Object-safe AwaitEdgeSettler/AwaitEdgeWriter (non-generic deps/observer);
  shared resume_parent_for_await_edge helper (one impl); pure resolver.
- B2 arch-exempt annotation + SubagentSpawnDeps ripple; config_file.rs
  SubagentSection (deny_unknown_fields); binding-ref helpers pub(crate);
  ironclaw_turns::IdempotencyKey named; FileType::Directory filter.

Also updated the design doc §3 / PR0 row to match the BlockedDependentRun reuse.
No change to approved product behavior; all fixes are correctness/plumbing.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

Author: henrypark133