feat: implement MCP proxy for OpenClaw tool access in contractor agent

Complete the MCP tool call chain:
- contractor-agent bridge exposes /mcp/execute endpoint for tool callbacks
- openclaw-mcp-server.mjs proxies OpenClaw tool defs to Claude as MCP tools
- sdk-adapter passes --mcp-config on first turn with all OpenClaw tools
- tool-test plugin registers contractor_echo in globalThis tool handler map
- agent-config-writer auto-sets tools.profile=full so OpenClaw sends tool defs
- Fix --mcp-config arg ordering: prompt must come before <configs...> flag

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

docs/claude/IMPLEMENTATION.md

@@ -372,6 +372,25 @@ export type ContractorBridgeModelRequest = {
 
 Exact fields depend on how OpenClaw provider/model requests are exposed in runtime.
 
+## Implementation notes from probe testing
+
+See BRIDGE_MODEL_FINDINGS.md for full details. Key constraints that affect implementation:
+
+- The sidecar MUST support SSE streaming (`stream: true` always sent by OpenClaw). Non-streaming
+  responses cause OpenClaw to drop assistant history from subsequent turns.
+- OpenClaw sends the full system prompt (~28K chars) rebuilt on every turn. The input filter
+  must strip this and extract only the latest user message.
+- User message format from OpenClaw: `[Day YYYY-MM-DD HH:MM TZ] message text`
+- Claude session continuation uses UUID from `@anthropic-ai/claude-agent-sdk` `message.session_id`.
+  Resume via `options: { resume: sessionId }`.
+- Custom model is registered via `openclaw.json` provider config, not a plugin SDK call.
+  The install script must write the provider entry and set `baseUrl` to the sidecar port.
+
+## Revised milestone order
+
+Milestone 3 (bridge model) is moved before Milestone 2 (CLI) because the bridge sidecar is the
+load-bearing component. Everything else depends on confirming it works end-to-end.
+
 ## Suggested implementation order
 
 ## Milestone 1, project skeleton