Derive runs purge context from the command instead of context.Background()

[itseffi]

Problem

newRunsPurgeCommand's RunE used context.Background() — the only RunE in
internal/cli that did. Its two siblings in the same file (runs.go:39, runs.go:61)
use signalCommandContext(cmd), which derives from cmd.Context() when set and wraps
it in signal.NotifyContext.

So runs purge discarded both signal handling and any embedder-supplied context. That
context is not decorative — it flows into cancellable work, including
daemon.LoadRunLifecycleSettings(ctx) and globaldb.Open(ctx, ...), so a purge blocked
on the database could not be interrupted with Ctrl-C.

Fix

Two lines in internal/cli/runs.go: context.Background() becomes
signalCommandContext(cmd) with defer stop(), and the now-unused context import is
dropped. No other behaviour changed.

Test

Adds TestRunsPurgeCommandHonorsCanceledCommandContext. It isolates HOME to a temp
dir and builds the home layout so the purge would otherwise succeed, sets an
already-cancelled context via cmd.SetContext(ctx), runs runs purge, and asserts
errors.Is(err, context.Canceled) and that no "purged" line was printed.

Red-green confirmed: with context.Background() restored the test fails with
runs purge error = <nil>, want context.Canceled.

Verification

make verify passes end to end — golangci-lint fmt, golangci-lint run with zero
issues, 3666 tests, and the build. Also verified merged together with #(the replay fix)
on an integration branch: 3669 tests, gate green.

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