fix(stack): rebase complete dependent PR trees

[patroza]

What Changed

• discover the complete same-repository PR tree rooted at fork/changes
• cascade rewrites parent-first through overlay children and deeper dependent PRs
• recover stale children from their parent PR force-push history
• preserve intentional parent branches in fork:stack update <pr> instead of retargeting children to fork/changes
• isolate conflicts to the failed PR subtree
• document dependent-PR behavior in the fork workflow and agent instructions

Why

The stack cascade only selected PRs directly targeting fork/changes. Registered overlays were rebased, but child PRs such as #98 were left on rewritten history and became conflicting. The manual update helper also hardcoded fork/changes, making it unsafe for overlay children.

Downstream Fork Relationship

Base: fork/changes. Affected area: fork stack automation and documentation. No dependency on another open PR.

Verification

• vp check (0 errors)
• vp test run scripts/fork-stack.test.ts scripts/rebase-pr-stack.test.ts (44 tests)
• (cd scripts && ../node_modules/.bin/tsgo --noEmit)
• temporary-git integration fixtures cover overlay → child → grandchild cascading and stale child recovery after a missed parent force-push
• live validation: repaired fix(discord-bot): handle Discord prompt edits cleanly #98 onto fork/discord using recovered parent tip 3387d32025f7; GitHub now reports MERGEABLE/CLEAN with one child commit

Checklist

• This PR is small and focused
• I explained what changed and why
• UI screenshots are not applicable
• Animation/interaction video is not applicable