docs(reborn): remove superseded subagent-durability sub-spec

The 2026-06-08 durability sub-spec is fully replaced by the 2026-06-23
subagent thread harness design (filesystem await-edge model). Removed to keep
a single source of truth for the durable delivery layer.

Deliberately kept: docs/reborn/subagent-spawn/ (shipped blocking-mode spawn,
referenced by 3 files), the compaction design, and the planner-subagent spec —
all document live shipped code this design builds on, not the superseded layer.

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

Author: henrypark133