updater: `CacheTTL = 0 // force fresh check` is a no-op — `update` can report a stale version

[IceRhymers]

Found while planning #217.

Bug

All three launchers' update blocks do:

cfg := buildUpdaterConfig()
cfg.CacheTTL = 0 // force fresh check

But Check() opens with (internal/core/updater/updater.go:63-65):

if cfg.CacheTTL == 0 { cfg.CacheTTL = 24 * time.Hour }

And buildUpdaterConfig already sets CacheTTL: 24 * time.Hour. So the line is a total no-op, not a partial one: update reads a <24h-old cache and can silently report a stale version. The explicit, user-invoked "check for updates" command does not necessarily check.

Fix sketch

CacheTTL == 0 is overloaded: it means both "unset, use the default" and "bypass the cache". Disambiguate — e.g. a negative TTL, an explicit ForceRefresh bool on Config, or a CheckFresh(ctx, cfg) entry point. Whichever way, update must actually hit the network.

Note this is a real behavior change (every update invocation gains a network call), which is why it was kept out of #217.

Also in scope — two README lines this falsifies

• README describes update as force-checking. It does not.
• README:1056: "Both suppress the startup check and disable the update subcommand." Only the DATABRICKS_NO_UPDATE_CHECK env var does. --no-update-check does not disable the update subcommand — the update block early-exits before parseArgs ever sees the flag.

Context

After #217 the runner is shared, so this is a one-line fix in one place. The TODO in internal/core/updater.RunUpdateCommand references this issue.