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

[hungmuha]

LGTM — nested-topology rendering is correct across fork/deep/cycle/round-trip cases with solid new test coverage. Two minor nits:

• nitsrc/stackGraph.ts:195 — displayChainFor no longer has a production caller after Stack.ts switched to displayTreeFor; only its definition, the interface member, the return-object entry, and StackGraph tests reference it. If not intended as retained API, remove it and its stale tests; otherwise note it is test-only.
• nitsrc/stackBlock.ts:116 — line() appends a branch-label suffix in backticks, but completedLines parses the branch key via the first backtick group. With showTitles (GitLab), a PR/MR title containing backticks makes the first group match the title fragment instead of the branch, so the next sync resolves the wrong key and strips the title inline code. Low likelihood/cosmetic, but the round-trip is lossy. Consider anchoring the branch-label match to end-of-line.

[klaemo]

oh this would be lovely