feat: Session templates — save, reuse, quick-start

[OneStepAt4time]

Summary

Allow users to save any completed/archived session as a reusable template for quick session creation.

Design

• Templates stored in ~/.aegis/templates/{id}.json
• Template contains: name, prompt, workDir, permissionMode, env vars, ttlMs
• "Save as Template" action on any session (active or archived)
• Templates shown in "New Session" flow as quick-start options
• Template CRUD via API

API Additions

Endpoint	Method	Purpose
/v1/templates	GET	List templates
/v1/templates	POST	Create template
/v1/templates/:id	DELETE	Delete template
/v1/sessions/:id/template	POST	Save session as template
/v1/sessions (create)	POST	New field: templateId

Data Model

interface SessionTemplate {
  id: string;
  name: string;
  prompt: string;
  workDir: string;
  permissionMode: string;
  env?: Record<string, string>;
  ttlMs: number;
  createdAt: number;
}

Depends On

• Session persistence (templates build on stored session data)

Spec

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

[OneStepAt4time]

Implemented via #996 and merged on main. Closing manually because auto-close keyword did not trigger.