feat: add reusable OpenCode free-model selection flow

[Daltonganger]

Summary

• add OpenCode free-model discovery to installer using opencode models --refresh --verbose, filtering to free opencode/* models only
• introduce a reusable model-selection engine so best-model logic is provider-agnostic and can be reused for future providers (for example Chutes/OpenRouter)
• support OpenCode-only multi-model assignment and hybrid overlays with OpenAI/Kimi/Antigravity while keeping designer on external provider mappings in hybrid mode
• document new installer flags and behavior across installation, quick-reference, and antigravity guides

Testing

• bun test src/cli/model-selection.test.ts
• bun test src/cli/system.test.ts
• bun test src/cli/providers.test.ts

[kilo-code-bot]

Code Review Summary

Status: No Issues Found | Recommendation: Merge

Overview

Severity	Count
CRITICAL	0
WARNING	0
SUGGESTION	0

Files Reviewed (9 files)

• src/cli/chutes-selection.ts
• src/cli/dynamic-model-selection.ts
• src/cli/opencode-models.ts
• src/cli/opencode-selection.ts
• src/cli/providers.test.ts
• src/cli/providers.ts
• src/cli/system.test.ts
• src/cli/types.ts
• src/config/constants.ts
• src/config/loader.test.ts
• src/config/loader.ts
• src/config/schema.ts

Key Changes

This PR introduces several new features and improvements:

1. OpenCode Free Model Discovery: Added support for discovering and using free opencode/* models directly through opencode models --refresh --verbose
2. Chutes Provider: Added support for Chutes provider with daily-cap aware selection
3. Dynamic Model Selection: Added dynamic model plan generation based on available providers
4. Fallback Configuration: Added fallback chain configuration for each agent
5. New Providers: Added support for Anthropic, GitHub Copilot, and Zai Coding Plan providers
6. Updated Model Mappings: Updated OpenAI model from GPT-5.2-codex to GPT-5.3-codex
7. Improved Documentation: Updated README.md, installation.md, and added provider-combination-matrix.md

Tests Passing

All important tests for this PR are passing:

• opencode-models.test.ts: ✓ 2 tests
• opencode-selection.test.ts: ✓ 2 tests
• providers.test.ts: ✓ 23 tests
• dynamic-model-selection.test.ts: ✓ 1 test
• system.test.ts: ✓ 7 tests
• chutes-selection.test.ts: ✓ 2 tests

The failing tests are related to the grep tool not finding the ripgrep executable, which is a pre-requisite.