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/crates/mock-anthropic-service/src/main.rs

@@ -0,0 +1,34 @@
+use std::env;
+
+use mock_anthropic_service::MockAnthropicService;
+
+#[tokio::main(flavor = "multi_thread")]
+async fn main() -> Result<(), Box<dyn std::error::Error>> {
+    let mut bind_addr = String::from("127.0.0.1:0");
+    let mut args = env::args().skip(1);
+    while let Some(arg) = args.next() {
+        match arg.as_str() {
+            "--bind" => {
+                bind_addr = args
+                    .next()
+                    .ok_or_else(|| "missing value for --bind".to_string())?;
+            }
+            flag if flag.starts_with("--bind=") => {
+                bind_addr = flag[7..].to_string();
+            }
+            "--help" | "-h" => {
+                println!("Usage: mock-anthropic-service [--bind HOST:PORT]");
+                return Ok(());
+            }
+            other => {
+                return Err(format!("unsupported argument: {other}").into());
+            }
+        }
+    }
+
+    let server = MockAnthropicService::spawn_on(&bind_addr).await?;
+    println!("MOCK_ANTHROPIC_BASE_URL={}", server.base_url());
+    tokio::signal::ctrl_c().await?;
+    drop(server);
+    Ok(())
+}