docs(reborn): implementation plan for subagent harness PR0 + PR1

Bite-sized TDD plan for the prerequisites (BlockedAwaitEdge variant,
wrap_untrusted_subagent_text visibility, subagent.v2_enabled flag) and the
durable await-edge walking skeleton (filesystem await-edge store + CAS
transitions + resolver + crash-recovery proof + flag-gated blocking delivery).
Grounded in verified signatures; corrects the design's implication that the
spawn depth floor is new (it already exists at subagent_spawn_port.rs:653).

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

Author: henrypark133