feat: squads init seeds agent files with structured frontmatter schemas

[kokevidaurre]

What

squads init should create agent files with proper frontmatter and structured body sections so the context system works out of the box.

Current behavior

Init creates minimal .md files without frontmatter. The context system can't determine roles, and agents get no structured instructions.

Expected behavior

Agent files created by init should follow this schema:

---
role: "worker"
squad: "my-squad"
provider: "anthropic"
model: "claude-sonnet-4"
trigger: "event"
cooldown: "1h"
timeout: 1800
max_retries: 2
---

# Agent Name

## Role
(one sentence)

## How You Work
(methods, patterns)

## Output
(what you produce, where)

## Constraints
(boundaries)

Also: priorities.md and goals.md should be created with frontmatter:

---
squad: "my-squad"
updated: "2026-03-24"
review_by: "2026-04-07"
owner: "my-squad-lead"
---

Acceptance criteria

• Agent .md files have frontmatter (role, squad, provider, model, trigger)
• Agent body has structured sections (Role, How You Work, Output, Constraints)
• priorities.md created with frontmatter
• goals.md created with frontmatter (Active/Achieved/Abandoned sections)
• SYSTEM.md created from built-in template with frontmatter
• company.md created with frontmatter