feat: Structured model preference & squad-level defaults

[bradygaster]

Summary

Closes #223 — Model & reasoning-level configuration not reliably applied by agents.

Working as Fenster (Core Dev).

Problem

The \model\ field in \AgentDefinition\ was a bare string. Charter generation output a flat *Model:* {value}\ line instead of the ## Model\ section that the charter-compiler expects (with **Preferred:**\ format). This meant model preferences set via SDK config were silently dropped during the parse → compile round-trip.

Changes

SDK types (\packages/squad-sdk/src/builders/types.ts):

• Added \ModelPreference\ interface: { preferred, rationale?, fallback? }\
• \AgentDefinition.model\ now accepts \string | ModelPreference\ (backwards compatible)
• Added \DefaultsDefinition\ with \model\ field for squad-level defaults
• Added \defaults?\ field to \SquadSDKConfig\

Builder validation (\packages/squad-sdk/src/builders/index.ts):

• Added \�ssertModelPreference()\ validator for string-or-object model fields
• Added \defineDefaults()\ builder function
• Updated \defineSquad()\ to validate defaults

Charter generation (\packages/squad-cli/src/cli/commands/build.ts):

• Charter output now emits a proper ## Model\ section with **Preferred:, **Rationale:, and **Fallback:**\ lines
• Applies \defaults.model\ when an agent doesn't specify its own model

Charter compiler (\packages/squad-sdk/src/agents/charter-compiler.ts):

• Now extracts \modelRationale\ and \modelFallback\ from ## Model\ section
• \ParsedCharter\ interface expanded with \modelRationale?\ and \modelFallback?\

Exports (\packages/squad-sdk/src/index.ts):

• Exports \defineDefaults, \ModelPreference, \DefaultsDefinition\

Tests: Updated \ est/build-command.test.ts\ and \ est/charter-compiler.test.ts\ — all 62+36 tests pass.

Usage

\\ s
// Per-agent (string — backwards compatible)
defineAgent({ name: 'edie', role: 'TS Eng', model: 'claude-sonnet-4' });

// Per-agent (structured)
defineAgent({
name: 'edie',
role: 'TS Eng',
model: { preferred: 'claude-sonnet-4', rationale: 'Best for type work', fallback: 'claude-haiku-4.5' },
});

// Squad-level default
defineSquad({
team: defineTeam({ ... }),
agents: [...],
defaults: defineDefaults({ model: 'claude-sonnet-4' }),
});
\\