fix(scripts): version-switch.sh says whether the start warned of plugins.toml - #3255

Description

scripts/version-switch.sh reported a plugin that a 0.46 → main upgrade turned back on as "on again, silently". Since #3239 (shortened in #3245) main warns at every start of a home that still has 0.46's plugins.toml and no [plugins] table in settings.toml (the state a 0.46 home is in after rig plugin disable there):

rig: rig no longer reads …/.rig/plugins.toml. Move its plugin list to [plugins] in …/.rig/settings.toml (see MIGRATING.md). The file was not changed.

so "silently" was wrong for the upgrade. It is still right for the downgrade: v0.46.0 knows nothing of [plugins] and says nothing.

The check now looks at the stderr of the turn just before it. When that turn named plugins.toml, the detail says so; otherwise it still says "silently". The check's result (XFAIL for both pairs) doesn't change.

Before (a release-PR run's summary, from a 0.46.0 → main run):

XFAIL  settings-upgrade    rig_basics::skills::SkillsPlugin is on again, silently
XFAIL  settings-downgrade  rig_basics::agents_md::AgentsMdPlugin is on again, silently

After, on the same logs:

XFAIL  settings-upgrade    rig_basics::skills::SkillsPlugin is on again; the turn before warned of plugins.toml
XFAIL  settings-downgrade  rig_basics::agents_md::AgentsMdPlugin is on again, silently

scripts/version-switch-expected.txt's comment for settings-upgrade no longer says "silently" either.

Changelog

None (a maintainer script's output text).

Migration

None.

Type of change

• Bug fix

Testing

• bash -n scripts/version-switch.sh.
• Not a full run: the script makes six agent builds, five of them cold (a few minutes each). Instead I ran the script's own check and plugin_off functions, taken from main's and this branch's script, on the recorded logs of an earlier full 0.46.0 → main run (its upgrade.err has the warning above, its downgrade.err doesn't). Output above: main says "silently" for both, this branch only for the downgrade.
• shellcheck: not run (not installed here, and CI doesn't run it).

Line deltas: script +6 / −3, no Rust.

Notes

• Choices left to the owner: none.

Try it (a full run, about half an hour of builds):

cd ~/code/rig && git fetch origin fix/version-switch-xfail-text && git worktree add --detach ~/.cache/claude-tmp/vs-13 FETCH_HEAD && cd ~/.cache/claude-tmp/vs-13 && scripts/version-switch.sh v0.46.0 HEAD --work ~/.cache/claude-tmp/vs-13-work --expected scripts/version-switch-expected.txt