boxel-cli: add boxel realm milestone command

[FadhlanR]

Summary

• Ports the milestone command from the standalone cardstack/boxel-cli repo into the monorepo as boxel realm milestone
• New command lives at packages/boxel-cli/src/commands/realm/milestone.ts, following the same pattern as realm history
• Backed by the existing CheckpointManager methods (markMilestone, unmarkMilestone, getMilestones) — no library changes needed

Commands

boxel realm milestone <local-dir>                            # list all milestones
boxel realm milestone <local-dir> --mark <ref> --name <n>   # tag a checkpoint as milestone
boxel realm milestone <local-dir> --remove <ref>            # remove milestone tag
boxel realm milestone <local-dir> --json                    # machine-readable output

<ref> accepts a 1-based index, 7-char short hash, or full hash (via existing findCheckpoint).

Test plan

• 18 new integration tests in tests/integration/realm-milestone.test.ts — all passing locally
• Existing checkpoint-manager unit tests (28) still pass
• pnpm tsc --noEmit clean

Closes CS-10626

🤖 Generated with Claude Code

[chatgpt-codex-connector]

💡 Codex Review

Here are some automated review suggestions for this pull request.

Reviewed commit: 21a6c246de

ℹ️ About Codex in GitHub

Your team has set up Codex to review pull requests in this repo. Reviews are triggered when you

• Open a pull request for review
• Mark a draft as ready
• Comment "@codex review".

If Codex has suggestions, it will comment; otherwise it will react with 👍.

Codex can also answer questions or update the PR. Try commenting "@codex address that feedback".

[chatgpt-codex-connector]

List all milestones instead of only latest 100

listMilestonesStep calls manager.getMilestones() without any way to override its internal cap (currently 100 checkpoints), so boxel realm milestone <local-dir> silently omits milestones on older commits in long-lived workspaces. Because this command is presented as listing milestones generally, users can miss existing milestone tags unless they happen to be in the newest 100 checkpoints; consider plumbing a limit through list mode (or removing the cap) so listing is complete/predictable.

Useful? React with 👍 / 👎.