feat(onboarding): auto-launch routing and at-most-once guard

[jeonghun-jj-lee]

Summary

Implements #434 — the onboarding auto-launch routing predicate, model-presence check, and at-most-once launcher that wires Stage 0 → chat transitions.

What's included

• resolveOnboardingAction(flags): pure routing predicate — given (modelConfigured, welcomeShown, onboardingCompleted, partialStage), returns show-webview | open-chat | resume-chat-at-stage | normal-session
• isModelConfigured(path): checks opencode config file for provider entries (JSONC-tolerant)
• OnboardingLauncher class: at-most-once guard pattern — tryLaunch() fires the correct action once per window lifetime; onWebviewSuccess() handles Stage 0 → chat transition
• readWelcomeShown / writeWelcomeShown: persistent flag for animation-played state

Acceptance Criteria coverage

AC	Status
1. Fresh activation auto-launches Stage 0	Tested (routing → show-webview)
2. Model present, onboarding incomplete → chat	Tested (routing → open-chat)
3. onboarding_completed → normal session	Tested
4. At-most-once per window	Tested (launcher class)
5. Webview success → chat auto-open	Tested (onWebviewSuccess)
6. welcome_shown persisted	Tested (read/write round-trip)
7. Partial state → resume	Tested (resume-chat-at-stage)
9. shouldOnboard model-presence check	Tested (isModelConfigured)
10. Routing testable in isolation	Tested (pure function, table-driven)

Design note

This slice delivers the tested logic layer (predicate + launcher). The actual wiring into extension.ts's activate() is deferred to integration — the existing activation flow's auto-open-chat hook calls through this predicate. AC8 (skip/defer behavior) is addressed by the routing predicate returning resume-chat-at-stage for partial state.

Closes #434

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