Lock down CLI-to-mock behavioral parity for Anthropic flows

This adds a deterministic mock Anthropic-compatible /v1/messages service,
a clean-environment CLI harness, and repo docs so the first parity
milestone can be validated without live network dependencies.

Constraint: First milestone must prove Rust claw can connect from a clean environment and cover streaming, tool assembly, and permission/tool flow
Constraint: No new third-party dependencies; reuse the existing Rust workspace stack
Rejected: Record/replay live Anthropic traffic | nondeterministic and unsuitable for repeatable CI coverage
Confidence: high
Scope-risk: moderate
Reversibility: clean
Directive: Keep scenario markers and expected tool payload shapes synchronized between the mock service and the harness tests
Tested: cargo fmt --all
Tested: cargo clippy --workspace --all-targets -- -D warnings
Tested: cargo test --workspace
Tested: ./scripts/run_mock_parity_harness.sh
Not-tested: Live Anthropic responses beyond the five scripted harness scenarios

rust/Cargo.lock

@@ -719,6 +719,15 @@ dependencies = [
  "windows-sys 0.61.2",
 ]
 
+[[package]]
+name = "mock-anthropic-service"
+version = "0.1.0"
+dependencies = [
+ "api",
+ "serde_json",
+ "tokio",
+]
+
 [[package]]
 name = "nibble_vec"
 version = "0.1.0"
@@ -1194,6 +1203,7 @@ dependencies = [
  "commands",
  "compat-harness",
  "crossterm",
+ "mock-anthropic-service",
  "plugins",
  "pulldown-cmark",
  "runtime",