feat: Session forking — create new session from parent context

[OneStepAt4time]

Summary

Allow users to fork any session (active or archived) into a new CC session with inherited context.

Design

• Fork creates a new CC session in the same workDir
• Prompt is seeded from the parent session's summary (uses existing GET /v1/sessions/:id/summary)
• Fork relationship tracked via forkedFrom field in session metadata
• Not a git fork — a fresh CC session with inherited context

API Addition

Endpoint	Method	Purpose
/v1/sessions/:id/fork	POST	Fork a session

Data Model

interface Session {
  // ... existing fields
  forkedFrom?: string;  // parent session id
}

UI

• "Fork" button on session actions (header, sidebar hover)
• SessionForkDialog — confirmation + optional prompt editing before forking
• Fork indicator in session metadata (shows parent link)

Depends On

• Session persistence (need stored sessions to fork from)
• Session summary API (used to seed fork prompt)

Spec

See docs/superpowers/specs/2026-03-28-dashboard-ux-p1-design.md — Section 2: Session Forking