feat: metadata-driven env injection and runtime dropdown UX overhaul

[wpfleger96]

Summary

Stacked on #783 (provider→runtime rename). Consumes the KnownAcpRuntime capability metadata fields added in Phase 2 and redesigns the runtime dropdown in the "Add agents" dialog.

Rust (env injection):

• Replace hardcoded GOOSE_MODE=auto injection with a metadata-driven default_env loop — new runtimes get their defaults by adding to the catalog, not touching spawn logic
• Inject GOOSE_MODEL via model_env_var for runtimes that don't support ACP model switching, so goose agents respect PersonaRecord.model even without a persona pack
• Same pattern applied to the model probe subprocess in agent_models.rs
• Remove #[allow(dead_code)] from the three now-consumed fields (supports_acp_model_switching, model_env_var, default_env)

TypeScript (runtime dropdown UX):

• Runtime dropdown defaults to "Use persona defaults" (unset) instead of auto-selecting the first available runtime
• Explicit runtime selection overrides ALL personas — no more silent bypassing when a persona has a stored preference
• Per-persona runtime badges on persona chips show the effective runtime, with warning styling when overridden or when a stored preference is unavailable
• resolvePersonaRuntime gains a forceOverride parameter and an isOverridden field on the result type — single source of truth for override decisions
• useEffectiveRuntimes hook extracted from AddChannelBotDialog for file size and testability
• RuntimeBadge extracted as a named component from inline IIFE
• RUNTIME_NONE_SENTINEL constant replaces magic string
• 22 unit tests covering all resolution cases, override modes, and isOverridden semantics