fix(stack): support mixed stack topology

[aryasaatvik]

Summary

Support mixed linear and parallel stack shapes without conflating merge path selection with PR stack-block rendering.

Changes

• Split StackGraph chain APIs into pathTo() for merge --auto --through and displayChainFor() for PR body rendering.
• Keep bounded auto-merge on the selected root-to-target path, avoiding unrelated roots or sibling branches.
• Preserve root-first full stack blocks for linear PR stacks.
• Add regressions for sibling branches, unrelated roots, and root PR stack-block rendering.

Tests

• bun run typecheck
• bun run test
• bun run format:check
• bun run lint
• bun src/cli.ts --help
• bun src/cli.ts sync --help
• bun src/cli.ts merge --help