feat: add activity log command

[abpai]

Summary

• Adds wildcard activity log command for browsing and displaying the activity timeline
• Introduces time-bucketing logic for grouping events into sessions
• Adds AI-powered summary prompt templates loaded from markdown files
• Includes scaffolding for the specify workflow and updated project docs/architecture

Test plan

• bun test passes (activity-log, time-bucketing, prompt-templates covered)
• bun run lint passes
• wildcard activity log displays expected output from a live session
• wildcard activity show still works as before