feat(tool): port openclaude context primitives

[Patel230]

Summary

Adopts four self-contained, stdlib-only context/robustness mechanisms from the OpenClaude coding agent (github.com/Gitlawb/openclaude), ported to idiomatic Go with full tests. None existed in hawk before.

1. internal/circuitbreaker — closed/open/half-open circuit breaker with cooldown (port of OpenClaude's auto-compact breaker: 3 failures -> cooldown -> half-open probe that a single failure re-opens, a single success closes). Time-injected, concurrency-safe. hawk's AutoCompactor already has a simpler consecutive-failure guard; this is the generalized primitive with cooldown + auto half-open retry.

2. internal/smartrouting — deterministic simple/strong turn classifier (port of OpenClaude smart routing): cheap model for trivial turns, strong for anything non-trivial, always fail-toward-strong so a misroute never degrades a hard answer. Pure function, word-boundary keyword matching.

3. internal/conversationarc — durable sidecar memory of goals/decisions/milestones/phase (init->exploring->implementing->reviewing->completed), persisted to .arc.json, with a byte-stable model-visible summary (volatile timestamps normalized so unchanged arcs don't rewrite).

4. internal/relevanceprune — token-budgeted context pruning that scores older messages by keyword overlap with the task context, keeps highest-relevance groups up to budget, and always preserves the recent tail + tool calls + errors.

All boundary-compliant (zero eyrie/client imports, stdlib-only). README adopted-capabilities table updated.

Verification

• 30+ new tests across the four packages
• go build ./..., go vet, full ./internal/{circuitbreaker,smartrouting,conversationarc,relevanceprune} suites green
• All pre-push boundary guards + govulncheck + full test suite green