D-6: SharedModels — introduce SessionSurface enum

[kirich1409]

Description

Add a SessionSurface enum and related types to SharedModels. Immutable after session creation; differentiates shell vs terminal-rendered agent vs chat-rendered agent. Foundation for all downstream routing (PaneManager, AgentSessionFeature, MainFeature).

Spec: Epic #250 §5 (Session surface model).

Scope

Add MacApp/Packages/SharedModels/Sources/SharedModels/SessionSurface.swift:

```swift
public enum SessionSurface: Equatable, Sendable, Codable {
case shell
case agentTerminal(AgentKind)
case agentDialogue(AgentKind)
}

public enum AgentKind: String, Equatable, Sendable, Codable, CaseIterable {
case claudeCode
// future: codex, aider, acpGeneric
}

public extension SessionSurface {
var isAgent: Bool { switch self { case .shell: false; default: true } }
var agentKind: AgentKind? { /* extract kind or nil / }
var isDialogue: Bool { / ... */ }
}
```

Extend AgentSession (value type in SharedModels) to include surface: SessionSurface — immutable let-property set at creation.

Codable: derive default coding; ensure roundtrip for all cases. For Tab.kind (SessionKind enum: .default / .userCreated) — keep untouched, surface is orthogonal.

Acceptance Criteria

• SessionSurface.swift with enum, AgentKind enum, public computed helpers.
• AgentSession extended with immutable surface: SessionSurface.
• Codable roundtrip test for .shell, .agentTerminal(.claudeCode), .agentDialogue(.claudeCode).
• Equatable works as expected (different surfaces on the same agent kind are distinct).
• AgentKind.allCases usable for enumerating supported kinds (CaseIterable).
• Swift 6 strict concurrency — all types Sendable.

Relationships

• Parent issue: EPIC: Dialogue — structured chat UI for agent sessions #250
• Blocks: D-20 (ClaudeCommandBuilder), D-21 (AgentSessionFeature Presentation), D-22 (PaneManager Tab.surface), D-24 (SettingsStore preferredClaudeSurface)