docs: draft managed delegation-fork lifecycle plan

[jbeda]

Plan / Interface

Draft architectural planning checkpoint for ADR 0283.

Tracking: #1596

Contents

• Corrects ADR 0283 to preserve opaque artifact handles rather than exposing child filesystem paths.
• Defines the managed fork allocation lifecycle, crash-only TTL semantics, cleanup quarantine/retry behavior, and configuration precedence.
• Adds the Split draft acceptance plan with 13 acceptance criteria.
• Updates the parallelism guide to match the current opaque artifact behavior.

Deliberate stop

The acceptance plan remains draft. Implementation must not begin until the five material decisions listed in the plan and #1596 receive human decisions, then this Plan / Interface PR is updated to proposed and reviewed/merged.

Validation

• bash .claude/skills/to-acceptance-plan/scripts/check-acceptance-plan.sh docs/acceptance/managed-delegation-fork-lifecycle.md
• bash .claude/skills/to-acceptance-plan/scripts/check-acceptance-plan-test.sh
• task docs
• git diff --check