docs: expand Claude contractor design

docs/claude/PLAN.md

@@ -23,6 +23,16 @@ Desired properties:
 
 Use a session proxy runtime rather than replaying full OpenClaw-managed context into Claude Code on every turn.
 
+Important implementation note: in OpenClaw this should be delivered as a **custom model**.
+
+Reason:
+
+- OpenClaw expects each agent to have a primary model
+- for this special contractor agent, the real primary intelligence is provided by Claude Code
+- therefore the most natural integration seam is a custom model that accepts OpenClaw messages, routes them into the mapped Claude Code session, and then injects Claude's response back into OpenClaw session state
+
+So although the architecture is conceptually a session proxy runtime, the concrete integration form inside OpenClaw should be a **bridge model** or **pseudo-model backend** for Claude-backed agents.
+
 Core idea:
 
 - map each OpenClaw session to a Claude Code session
@@ -30,6 +40,7 @@ Core idea:
 - resume or continue that Claude session
 - forward only the newest actionable message plus minimal metadata
 - avoid rebuilding full prompt history on every turn
+- return the Claude response through the custom model seam so it becomes part of normal OpenClaw agent flow
 
 This reduces:
 
@@ -137,6 +148,26 @@ This supports the idea that OpenClaw skills and tooling may be adapted into Clau
 Source:
 - https://code.claude.com/docs/en/plugins-reference
 
+## Bridge Model Direction
+
+The contractor-agent implementation should expose one or more plugin-provided custom model ids for OpenClaw agents to use as their primary model.
+
+For Claude phase 1, the working assumption is a model id in the shape of:
+
+- `contractor-claude-bridge`
+
+Behavior of this bridge model:
+
+1. receive the agent turn from OpenClaw
+2. identify the current OpenClaw session key and target contractor agent
+3. resolve or create the mapped Claude Code session
+4. filter OpenClaw-managed envelope/context so only the latest actionable message and minimal metadata are forwarded
+5. send the turn to Claude Code through the chosen ACP/session continuation path
+6. capture Claude's response
+7. inject the response back into OpenClaw through the model response path and session file flow
+
+This approach fits OpenClaw's requirement that every agent has a primary model while still allowing Claude Code to act as the true reasoning engine for contractor-backed agents.
+
 ## Initial CLI Plan
 
 Introduce a plugin-owned command: