Require an explicit default agent harness

[fenner888]

What changed

• require an explicit default harness choice during first-run onboarding
• offer the bundled Buzz Agent alongside ready external CLI runtimes
• explain that Fizz, Honey, Bumble, and other unpinned agents inherit this choice
• require a valid Buzz Agent provider configuration before onboarding can finish
• preserve existing saved defaults and per-agent runtime overrides

Why

When Devin was the only ready onboarding runtime, Buzz silently selected it as
the global default. The Welcome starter personas then inherited Devin, even
when the builder intended to create only a separate Devin-backed agent.

The default-harness page now uses catalog-projected capability facts to include
ready bundled runtimes and leaves the choice to the builder.

Validation

• 22 focused onboarding and Welcome unit tests
• 20 focused onboarding Playwright tests
• 3,520 desktop unit tests
• desktop Biome, file-size, text-size, pubkey, and TypeScript checks
• 1,675 Tauri unit tests plus 3 mixer diagnostics
• 568 mobile tests
• Rust formatting, Clippy, unit tests, and builds covered by the repository CI recipes
• macOS source-installer contract test for arm64 and x86_64

The just ci recipes were completed individually after the local Mac could not
hold both Rust target trees at once. The initial aggregate run stopped only on
No space left on device; no code or test assertion failed.

[devin-ai-integration]

✅ Devin Review: No Issues Found

Devin Review analyzed this PR and found no bugs or issues to report.