Megarepo: nested pin propagation for cross-repo PR stacks

Problem

When validating a cross-repo PR stack before merging upstream, downstream repos pin the upstream member (e.g., effect-utils) to a PR branch via mr config pin. However, repos with nested megarepos (e.g., dotfiles contains repos/private-shared and repos/livestore) fail in CI because:

1. The parent pins effect-utils to the PR branch ✓
2. CI runs mr apply --all which clones nested members (private-shared, livestore)
3. Nested members are cloned at their default branch (from the parent's lock)
4. Nested members' own megarepo.json still references effect-utils at main
5. Nested sync tries to clone effect-utils at main → store conflict with the parent's PR branch clone

Current tools

• mr config push-refs --all propagates refs from parent megarepo.json to nested members' megarepo.json through symlinks
• But local symlinks point to default branch worktrees, not PR branch worktrees
• So push-refs modifications go to the wrong branch

Desired behavior

When a parent megarepo pins a member to a non-default ref, mr apply --all should ensure nested members can resolve that member consistently. Options:

1. Ref propagation in apply: when cloning nested members, inject the parent's pinned ref into the nested member's config before nested sync runs
2. Store-level dedup: if effect-utils is already in the store at a non-default ref, allow nested sync to reuse it instead of requiring an exact ref match
3. Cascade pin workflow: mr config push-refs should be able to target PR branch worktrees (not just default branch symlinks)

Workaround

Merge upstream first (topological order), then downstream PRs validate against merged main.

Start by tracing mr apply --all and mr config push-refs --all, then inspect how nested megarepo.json files and store refs are resolved. Compare the three proposed approaches and determine which behavior lets nested members reuse the parent's pinned ref without a store conflict; done means cross-repo PR stacks validate in CI without the described workaround.