Support parallel delegation, fix circular detection, and fix Anthropic proxy URL

[parruda]

Summary

• Fix Anthropic API 404 with proxy base URLs: Patch Anthropic::Chat#completion_url to return v1/messages (relative path) instead of /v1/messages (absolute). The leading slash caused Faraday to discard the base URL path component when anthropic_api_base includes a path segment (e.g., https://proxy.dev/apis/anthropic). Fixes both sync and streaming requests since stream_url delegates to completion_url.

• Fix false circular delegation detection: Replace the shared delegation_call_stack array on Swarm with Fiber-local path tracking (Fiber[:delegation_path]). Each Async Fiber tracks its own delegation chain, so parallel fan-out (A→B, A→B) is no longer falsely flagged as circular while true cycles (A→B→A) are still caught.

• Add concurrent delegation context isolation: When multiple parallel tool calls target the same delegate, only the first call preserves context; subsequent concurrent calls auto-clear via a new clear_context: parameter on Agent::Chat#ask. Clearing happens inside the ask_semaphore to prevent race conditions between parallel callers.

• Fix ThreadError: Attempt to unlock a mutex which is not locked: Replace defined?(Sync) check in run_with_sync with Async::Task.current?. When already inside an async reactor, just yield directly — avoids creating a nested reactor whose Promise#wait triggers the mutex error.

• Add request URL to llm_api_request event: Include the actual Faraday request URL in the llm_api_request event payload, enabling verification that LLM requests are sent to the correct endpoint.

Test plan

• All 2224 existing tests pass (0 failures, 0 errors)
• New test: circular detection uses Fiber-local path
• New test: parallel delegation to same target is allowed
• New test: concurrent delegation forces context clear (uses real Async barrier)
• New test: sequential delegations preserve context normally
• Rubocop clean on all changed files

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