fix: Progressive runs ignore cancellation while persisting synthetic follow-up/finalization turns

[sam-saffron-jarvis]

What changed

• replaced context.Background() for progressive synthetic continuation/finalization persistence with a short derived context that honors the active progressive run's cancellation/deadline
• added progressiveSyntheticUserMessageContext to bound OnSyntheticUserMessage callbacks to 5 seconds at most, while still stopping immediately when the parent progressive context is canceled or times out
• added regression tests covering both synthetic continuation persistence and synthetic finalization persistence so these callbacks cannot block forever on a detached background context

Why this is high-value

Progressive runs persist synthetic user turns through callbacks that ultimately write to the session SQLite store. Those writes can block behind SQLite busy/retry handling. Before this change, those two persistence calls used context.Background(), so a timed-out run, canceled job, or Ctrl-C could stay stuck persisting synthetic turns long after the progressive run was supposed to stop.

This fix closes that shutdown hole on the real ask --progressive / progressive jobs path without changing the broader progressive control flow.

Validation

• gofmt -w cmd/progressive.go cmd/progressive_test.go
• go build ./...
• targeted regression tests passed:
  • go test ./cmd -run 'TestRunProgressiveSessionSyntheticContinuationPersistenceHonorsContext|TestAttemptProgressiveFinalizationSyntheticPersistenceHonorsContext|TestRunProgressiveSessionTimeoutDoesNotStartDetachedFinalizationAfterDeadline|TestRunProgressiveSessionFinalizesOnNaturalCompletion'
• git diff --check
• go test ./... was run twice; the suite still hits the pre-existing unrelated flaky failure in internal/tools (TestRunAgentScriptTool_TimeoutKillsGrandchildren) when the entire repo is run together, but that test passes when rerun in isolation via go test ./internal/tools -run TestRunAgentScriptTool_TimeoutKillsGrandchildren -count=1 -v

[SamSaffron]

Applied to the working tree locally (along with #789/#790/#791 as a combined cancellation-correctness pass); will land in a direct commit.