fix: load ForgeFlow in native child sessions

[BakerSean168]

Summary

• register ForgeFlow itself through pi-subagents' required-child-extension registry for every Pi session
• make forgeflow/* virtual models available in native foreground, detached, nested, and recovery children
• dispose the required-child registration on session shutdown
• avoid hard-coding ForgeFlow installation paths in Pi operator profiles
• document why local foreground children need this explicit registration

Root cause

pi-subagents foreground children intentionally do not load the parent's ambient extensions. After #110, the parent could resolve forgeflow/reviewer, but a reviewer child failed with Model "forgeflow/reviewer:high" not found because the ForgeFlow extension that registers virtual models was absent from the child.

Verification

• npm run check: 47/47 passed
• npm pack --dry-run: passed
• git diff --check: passed
• lifecycle test proves the required-child registration owns the parent session and is released at shutdown