feat(stack): render stack blocks as nested topology

[aryasaatvik]

Summary

Render managed Stack blocks as nested Markdown lists so the visible widget matches the PR/MR target-branch topology. The block still preserves landed history above the open stack, but open siblings from stale flat blocks are no longer mistaken for history.

Examples

Linear stack with the middle PR current:

- #101 `stack-a`
  - **#102** 👈 current `stack-b`
    - #103 `stack-c`

Forked stack with siblings under the same parent:

- #201 `stack-root`
  - #202 `storage`
    - #204 `checks`
    - #205 `agent-tools`
  - **#203** 👈 current `presets`
  - #206 `docs`

Landed history remains above the open stack:

- #90
- #101 `stack-a`
  - **#102** 👈 current `stack-b`

Validation

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