refactor(terminal): replace shell-integration polling with event-based wait

[edelauna]

Summary

Track moving VS Code terminal shell-integration startup waiting from bounded polling (p-wait-for) to vscode.window.onDidChangeTerminalShellIntegration in a focused follow-up PR.

This was prototyped in #277 but should be separated because it affects every VS Code integrated-terminal command, not only profile overrides. The narrower cmd.exe compatibility path can remain in #277.

Motivation

An event-based wait can react immediately when shell integration becomes available and avoid polling during startup. The existing bounded polling path is simpler and should remain until the event-driven lifecycle is proven independently.

Risks to cover

• Missed-event race if shell integration becomes available between the initial property check and listener registration.
• Listener and timeout disposal on success, timeout, cancellation, and terminal disposal.
• Events for unrelated terminals.
• VS Code compatibility and mocks.
• Slow-start shells and remote environments such as Windows, WSL, SSH, and dev containers.

Acceptance criteria

• Subscribe before relying on event delivery and re-check terminal.shellIntegration after listener registration.
• Dispose the listener and timer on every completion path.
• Add focused tests for already-available integration, availability after subscription, availability during subscription setup, timeout cleanup, and unrelated-terminal events.
• Validate at least Windows and one slow-start or remote-shell scenario.
• Keep fallback behavior conservative: do not automatically replay a command after shellIntegration.executeCommand() has been called unless non-submission is definitive.

Context

Follow-up from #277 review.