feat: support configuring a default_agent across all API/user surfaces

[elithrar]

This PR supports defining a default_agent in opencode config (TUI, CLI, desktop, GitHub) so that opencode uses that from the outset. It also expands existing cases where users can provide an --agent flag to warn if their specified agent isn't found and fall back to "build" (existing behaviour, just not explicit to a user).

Changes

• adds default_agent field to config schema (Config.Info)
• adds default field to Agent.Info schema so clients (TUI, desktop) know which agent is the default
• adds Agent.defaultAgent() helper that resolves the configured default agent
• updates all hardcoded "build" fallbacks to use Agent.defaultAgent()
• updates TUI to select the default agent on initialization (was just picking first agent)
• adds agent input to GitHub Action for per-workflow override
• updates documentation for config and GitHub Action
• regenerates SDK types

Validation

• only primary agents can be set as the default (not subagents like explore)
• can be a built-in agent (build, plan) or a custom agent defined in config
• invalid/missing agents log a warning: default agent "{name}" not found. Falling back to "build"

Usage

{
  "$schema": "https://opencode.ai/config.json",
  "default_agent": "plan"
}

# GitHub Action
- uses: sst/opencode/github@latest
  with:
    model: anthropic/claude-sonnet-4-20250514
    agent: docs  # overrides the config default

[elithrar]

works in the TUI (default_agent: "triage")

[elithrar]

somehow I missed #5313 - I think I’ve implicitly superseded that PR here though as I covered all API surfaces (incl. github)

[observerw]

sweet!