Agent abstraction Phase B: per-session selection plumbing

[dhilgaertner]

Summary

Phase B of the agent-abstraction work scoped by #150. Add agentKind to the Session model, expose a default in AppConfig, and ship the per-session selector UI — wired up but disabled until Phase C adds a second agent.

Context

Full design in docs/agent-abstraction-spec.md §5, §12 Phase B.

Per the locked architectural decisions (§Executive Summary), the Manager tab stays pinned to Claude Code — no selector for Manager.

Checklist

• Add agentKind: AgentKind = .claudeCode to the Session model with Codable defaulting (backward compatible with existing persisted sessions)
• Add defaultAgentKind: AgentKind to AppConfig; render picker in SettingsView (only .claudeCode selectable until Phase C)
• Session-creation flow (SessionService, workspace wizard, New Session dialog) accepts and persists agentKind
• Manager-session construction hardcodes .claudeCode; UI hides the selector for Manager
• Sidebar adds an agent-icon slot (Claude icon only for now) in SessionListView
• Session-detail header displays Agent: <displayName> label (read-only, no switcher)
• Selector is exercised in code paths even while only one agent exists, so Phase C flips a flag rather than adds new plumbing

Non-goals

• No Codex implementation (Phase C)
• No dropdown options beyond Claude Code

Parent: #150 · Blocked by: #166 (Phase A) · Blocks: #168 (Phase C)