Discovery Engine — Phase 2: nav-sweep (deterministic imperative-nav traversal)

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Part of the Discovery Engine epic #125. Addresses the imperative-nav half of #120.

Scope — deterministic nav traversal (hybrid)

Button/router-nav SPAs expose no <a href>, so route discovery falls to the LLM per-page (budgeted, no global memory) → shallow crawls. Add a deterministic nav backbone that guarantees every nav entry point is traversed once.

• Gather (deterministic): scan inChrome nav-landmark clickables (nav, [role=navigation], [role=menu], sidebars) into a global, label/target-deduped nav checklist (crawl-wide, not per-page).
• Judge (existing LLM, no prompt change): unvisited nav items become high-priority candidates in the existing findUnexploredElements/planUnexploredElements flow.
• Fallback (deterministic): items the LLM skips stay on the global checklist → eventually clicked. Guard with SKIP_AUTO_DISCOVERY (logout/destructive).
• Visit via the existing executor (keeps ingest trigger-correlation) → url-change → enqueue route.

Constraints

• No prompt/snapshot change; additive; global-dedup prevents navbar re-click spam; guarded against logout.
• Generic (semantic landmarks, framework-agnostic); apps without semantic nav fall back to the existing LLM exploration.

Commits (granular)

Global nav-checklist state → inChrome gather → dedup keying → guard (SKIP_AUTO_DISCOVERY) → priority wiring into unexplored flow → executor-reuse visit → url-change enqueue → deterministic fallback → tests.