perf(bun): cache plugin versions with TTL to avoid reinstalling on every startup

[xammen]

Summary

• Fixes slow startup (~21 seconds) caused by reinstalling plugins on every launch
• When plugins use @latest, the cache now tracks timestamps and reuses cached versions for 24 hours
• Reduces plugin loading time from ~21s to <1s

Problem

The cache check compared 'latest' against the resolved version (e.g., '2.12.2'), which never matched:

if (parsed.dependencies[pkg] === version) return mod  // 'latest' !== '2.12.2'

Solution

• Added _timestamps field to track when packages were installed
• For @latest requests, reuse cached version if installed within 24 hours
• Still allows periodic updates while avoiding redundant reinstalls