D-25: Open as Terminal / Open as Dialogue — cross-surface resume via --resume

[kirich1409]

Description

Implement the cross-surface sibling-tab action — lets a user open a running Claude session in the opposite surface while preserving conversation context via claude --resume <session_id>.

Spec: Epic #250 §5 (cross-surface resume semantics).

Scope

Action surface (MainFeature + TabFeature)

• Menu item on active session: Open as Terminal (⌘⇧T) or Open as Dialogue (⌘⇧D) — visible based on current surface.
• Action dispatches AgentOrchestratorFeature.openAsSibling(currentTabID:targetSurface:).

Orchestrator logic

```swift
func openAsSibling(currentTabID: UUID, targetSurface: SessionSurface) {
// 1. Look up active session_id from current tab's AgentSessionFeature / AgentChatFeature state.
// 2. Create a new Tab with targetSurface, same worktree, same workingDirectory.
// 3. In the new tab's AgentSessionFeature.startAgent flow — pass sessionIDForResume.
// 4. ClaudeCommandBuilder emits --resume <UUID> + surface-specific flags.
// 5. Old tab remains — user can close it or keep both open in parallel.
}
```

Extraction of session_id

• For .agentDialogue source: from AgentChatFeature.State.sessionInit.sessionID.
• For .agentTerminal source: terminal doesn't structurally know session_id — read it from claude output by parsing a system/init line OR from the Claude Code session dir (~/.claude/projects/<hash>/). Prefer reading a sidecar file written by Claude (if available), else parse the first JSONL line after a one-shot claude status --output-format json probe.
• If session_id cannot be determined (very old Claude CLI, session too short) — fall back: open new session without --resume; warn user in UI.

UX

• Spinner / toast during the 1-2 seconds of sibling creation.
• On success: focus switches to new tab.
• On failure: toast "Couldn't open as {surface}. Starting new session instead." + new tab with fresh session.

Acceptance Criteria

• Menu items present with correct shortcuts; visible on active session of appropriate surface.
• Dialogue → Terminal: new tab opens with --resume <session_id>, shows prior transcript on first interaction (verified manually with live CLI or scripted).
• Terminal → Dialogue: same, reversed direction.
• Session_id detection fallback works when detection fails (new session, warning).
• Unit test: given a session_id, the builder produces --resume <id> + correct surface flags.
• Integration test (mocked PTY): both directions work without real network.
• A11y: menu items keyboard-reachable, announced meaningfully by VoiceOver.

Relationships

• Parent issue: EPIC: Dialogue — structured chat UI for agent sessions #250
• Blocked by: #D-20 (ClaudeCommandBuilder with --resume), #D-21 (Presentation), #D-22 (Tab.surface), #D-11 (lifecycle)
• Blocks: #D-27 (a11y)