feat: thin output-surface library — uniform agent API for Sheets, Slack, Gmail, WhatsApp

[agents-squads]

Problem

Agents currently reach output surfaces through ad-hoc calls:

• Google Sheets: direct gws CLI invocation
• Slack: direct bin/squads-slack invocation
• Gmail/WhatsApp: no pattern exists yet

This makes it hard to add new surfaces, test delivery, or retry on failure. There's no single place to observe what was delivered and when.

Proposal

A thin output-surface library (src/lib/output-surfaces.ts) that agents call with a uniform interface:

```ts
// Agent calls one function, library routes to the right surface
await deliver({
surface: 'sheets', // or 'slack' | 'gmail' | 'whatsapp'
content: { ... },
destination: { spreadsheetId: '...', range: 'A1' },
});
```

Surfaces to support

Surface	Status	Implementation
Google Sheets	proven (gws CLI)	wrap existing gws drive +write pattern
Slack	proven (bin/squads-slack)	wrap existing script
Gmail	untested	gws gmail send or equivalent
WhatsApp	not started	research Twilio vs WhatsApp Business API

Why this matters

Paying customers are non-terminal — they don't run the CLI. They receive outputs where they already work. The output-surface library is the delivery layer that makes "customer gets value without opening a terminal" possible.

Acceptance criteria

• deliver() function accepts surface + content + destination
• Sheets and Slack surfaces working end-to-end
• Gmail surface working (send digest to customer email)
• Error and retry handling per surface
• At least one surface tested with a real customer delivery