Skills and plugin sync can skip a redundant round on startup

What

Both the skills-sync and plugin-sync processes now accept a { startup } option. When Claude Code starts up (and isn't in cloud/XD mode, and no sync was force-requested), each of these now checks a saved timestamp to see whether a sync round already completed recently.

• If the last skills sync finished within CLAUDE_CODE_SYNC_REUSE_WITHIN_MS milliseconds and the rate-limit "bucket" guard hasn't refused it, the sync round is skipped entirely and a skills_sync_round_reused event is logged.
• Plugin sync gained an equivalent freshness check; if a recent round is found, it reuses that round instead of re-downloading and re-listing plugins, logging a plugins_sync_round_reused event.

Why

This avoids doing redundant sync work (downloading and re-listing skills or plugins) every time Claude Code starts, when a sync already happened moments ago, making startup faster and cheaper without affecting freshness.