H4.6: Custom automation rules

[SebastianElvis]

Overview

A rule engine for "when X, do Y":

• "When PR approved + CI passes → auto-merge"
• "When issue labelled bug → auto-start session"
• "When session fails 3x → notify Slack"
• "When token usage > $5 → pause and ask"

Tasks

• Design rule data model — trigger (event + conditions) → action (command)
• Build rules engine — evaluate rules against session event stream
• Build rules UI — create/edit/delete rules