Follow-up to #239

#239 retained default_agent in the config schema as a deprecated optional string while the one-shot migration (packages/opencode/src/config/migrate-default-agent.ts) propagates. Once v0.2.14 has shipped and most users have launched the app once (so the migration has run), the schema field should be removed.

Acceptance criteria

• packages/opencode/src/config/config.ts no longer defines default_agent in the config schema.
• packages/opencode/src/config/migrate-default-agent.ts and its test are deleted.
• The migration call site in loadGlobal is removed.
• Agent.defaultAgent() no longer reads c.default_agent (the gentle-fallback branch added in refactor(agent): remove visible primary-agent mode picker (#239) #242 review can simplify back to a single line that returns the first visible primary).
• Release notes for the version that ships this removal: configs that still contain default_agent after this release will fail validation; users on very old versions need to launch v0.2.13 once first to migrate.

Known minor cleanup from #242 review

• Draft pill rendering — packages/app/src/components/prompt-input.tsx:703 (renderEditor) still calls createPill(part) for part.type === "agent". With the extractPromptFromParts fix in refactor(agent): remove visible primary-agent mode picker (#239) #242, restored prompts no longer produce that branch in practice, but the dead-code branch can be removed when the schema field is removed for clean removal of all agent-pill plumbing.

When to do this

After v0.2.14 has been the latest release for at least two weeks, AND the project currently has <20 internal users (so the chance of an old config still containing the field is low). If user count grows significantly before that point, defer further.