feat(hermes): add chat model picker

[turingcat]

Summary

• add a model picker to both chat composer surfaces for built-in Hermes and custom Hermes profiles
• resolve each profile's actual HERMES_HOME, current model, provider endpoint, and model catalogue without falling back to the default profile
• persist model selections to model.default while preserving provider credentials, endpoint settings, and unrelated YAML sections
• defer reconnects until an active turn finishes and mark other sessions on the same launch-time config as stale
• keep Tauri and server transports in parity, with localized feedback and focused regression coverage

The picker is synthetic only while Hermes does not advertise its own ACP model option. If a future adapter provides one, the agent-owned option takes precedence.

Closes #776

Verification

• vitest run src/hooks/use-hermes-model-option.test.tsx src/components/chat/hermes-model-option-wiring.test.ts — 13 passed
• eslint on all changed TypeScript/TSX files
• tsc --noEmit --pretty false
• cargo test --features test-utils --lib commands::acp::tests — 246 passed
• cargo check
• cargo check --no-default-features --bin codeg-server
• cargo clippy --all-targets --features test-utils -- -D warnings
• cargo clippy --no-default-features --bin codeg-server --lib -- -D warnings