feat(wire): integrate openclaude context primitives

[Patel230]

Summary

Wires all four OpenClaude-adopted primitives (from PR #252) into hawk's running system.

circuitbreaker -> auto-compaction

• AutoCompactor (internal/engine/compact_auto.go) now uses the circuitbreaker package instead of a plain consecutive-failure int: adds the missing cooldown + half-open auto-retry (3 failures -> cooldown window -> half-open probe that a single failure re-opens). CompactConfig gains Cooldown (default 5m). Existing TestAutoCompactor_CircuitBreaker preserved (updated to the new API, same behavior).

relevanceprune -> compaction

• New RelevancePruneStrategy (internal/engine/compact_relevance.go) registered in the compaction registry between smart and truncate. Scores older messages against the last user turn, keeps the highest-relevance groups up to budget, preserves recent tail + tool calls + errors; degrades to truncation.

smartrouting -> per-turn model selection

• Settings.SmartRouting *smartrouting.Config (settings.json smart_routing), wired via LifecycleService.SetSmartRouting, applied in the stream model-cascade block (stream.go) — cheap simple model for trivial turns, strong otherwise, fail-toward-strong.

conversationarc -> durable session sidecar

• Session gains SetArc/Arc; loaded on session open (cmd/chat.go), saved on session close (cmd/chat_commands_session.go), and the byte-stable Summary() injected into the system prompt (stream.go buildTurnOptions) — empty arcs inject nothing.

Verification

• New tests: circuit-breaker cooldown/half-open, relevance strategy + adapter, smartrouting reroute, session arc accessors + IsEmpty
• go build ./..., go vet, full engine/config/cmd/conversationarc/circuitbreaker/smartrouting/relevanceprune suites green
• Boundary guard + govulncheck + full pre-push test suite green