sbx watch: new release v0.42.1 (baseline v0.39.0)

[github-actions]

Docker Sandboxes (sbx) released a new version.

Pinned baseline	v0.39.0
Latest release	v0.42.1

Why this matters

The planned --sandbox-mode sbx isolation backend shells out to the sbx CLI.
Because sbx is pre-1.0 and has broken its CLI before, every release is a potential
break of that integration.

What to check

• Read the release notes for changes to sbx run / its flags.
• Note any new or removed breaking changes (e.g. the deprecated sbx run <name> form being removed).
• Decide whether sbx is now mature enough to promote --sandbox-mode sbx from experimental.
• When reviewed, bump .github/sbx-watch-baseline.txt to v0.42.1 to silence this watch until the next release.

Latest release notes (truncated)

## What's New

### Bug Fixes
- Fix HTTP/2 upstream responses without bodies being incorrectly framed as chunked by the sandbox proxy.

---

_{Opened automatically by .github/workflows/sbx-watch.yaml. See docs/plans/Sandbox-Isolation-Modes.md.}

[NawaMan]

Reviewed v0.39.0 → v0.43.0 (v0.42.0, v0.42.1, v0.43.0 — the latest stable at review time, one ahead of what opened this issue).

CLI/config breakage since baseline

• v0.42.0: deprecated sbx run <kit-name> --kit <kit-ref> in favor of sbx run <kit-ref> as the agent positional. Another CLI-surface change on top of the earlier sbx run <name> → sbx run --name <name> and sbx policy set-default → sbx policy init breaks already called out in the plan doc.
• v0.42.0: --publish's default flipped from dual-stack to tcp4-only (breaking for anything relying on the old default).
• v0.43.0: shareSkills → skills in sbxenv.yaml; MCP OAuth client secret storage key format changed (an old secret silently stops being read).

Other notable findings

• v0.42.0 fixed two real security vulnerabilities (arbitrary host command execution via D-Bus, an OAuth callback-port hijack) — still finding/fixing that class of bug pre-1.0.
• Still v0.x — hasn't crossed the 1.0 milestone this watch is keyed on.

Decision

No code shells out to sbx yet (--sandbox-mode sbx is designed-but-deferred per docs/plans/Sandbox-Isolation-Modes.md), so none of this breaks anything today. But it's the exact pattern that keeps sbx experimental and last in the rollout order — continued breaking changes to the CLI/config surface, plus fresh security fixes, this close to baseline. Not promoting; staying deferred. Plan doc updated with these findings.

Baseline bumped to v0.43.0 (the latest reviewed, not just v0.42.1, to avoid an immediate re-fire tomorrow for content already covered here).