fix: new agents default to configured model instead of hardcoded groq

[chrisyoung2005]

Summary

• Built-in templates (`General Assistant`, `Code Helper`, `Researcher`, `Writer`, `Data Analyst`, `DevOps Engineer`) all hardcoded `provider = "groq"` / `model = "llama-3.3-70b-versatile"` in their `manifest_toml` strings
• The spawn wizard also initialized `spawnForm.provider = 'groq'` as a fallback default before the async `/api/status` fetch returned

The kernel's `default_model` overlay in `spawn_agent_with_parent` only activates when `provider`/`model` are empty or `"default"`. Hardcoded values bypassed the user's configured default entirely, causing new agents to boot with Groq regardless of what the user had set.

Changes

• `agents.js`: replace hardcoded `provider = "groq"` / `model = "llama-3.3-70b-versatile"` with `provider = "default"` / `model = "default"` in all six built-in template `manifest_toml` strings
• `agents.js`: change `spawnForm` initial defaults from `'groq'`/`'llama-3.3-70b-versatile'` to `'default'`/`'default'` so a slow or failed `/api/status` fetch does not silently override the user's intent

The backend kernel logic is correct and unchanged — it already handles `"default"` properly, including auto-detection when no provider is explicitly configured.

Testing

• `cargo clippy --workspace --all-targets -- -D warnings` — not verified locally: `openfang-runtime` has a pre-existing build failure on `main` (`mcp.rs:310` non_exhaustive struct, tracked in Fail to compile: cannot create non-exhaustive struct using struct expression #926/Not compiling #959) unrelated to this change
• `cargo test --workspace` — blocked by same pre-existing build failure
• Change is JavaScript-only; no Rust code modified
• Live integration tested: new agent created via wizard and template both boot with configured Ollama default instead of Groq

Security

• No new unsafe code
• No new dependencies
• No capability changes

Fixes #967