feat(coding-agent/slash-commands): add /retry command

[hansihe]

Summary

• Adds a /retry slash command that manually re-attempts the last failed or aborted assistant turn.
• Implements AgentSession.retry(): removes the failed assistant message, resets the retry budget, and reschedules the agent to continue.
• No-ops (with a "Nothing to retry" status) when the agent is busy or the last message isn't a failed/aborted assistant turn.

Motivation

Auto-retry handles transient failures during a turn, but once the budget is exhausted (or a turn is aborted) there was no first-class way to ask the agent to try again without manually editing state. /retry gives users a one-shot escape hatch with a fresh retry budget.

Behavior

• Guards against re-entry while isStreaming, isCompacting, or isRetrying.
• Only acts on a trailing assistant message whose stopReason is "error" or "aborted".
• Mirrors the auto-retry path: drops the failed message via replaceMessages, resets #retryAttempt, then schedules a continue.