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/MOCK_PARITY_HARNESS.md

@@ -0,0 +1,35 @@
+# Mock LLM parity harness
+
+This milestone adds a deterministic Anthropic-compatible mock service plus a reproducible CLI harness for the Rust `claw` binary.
+
+## Artifacts
+
+- `crates/mock-anthropic-service/` — mock `/v1/messages` service
+- `crates/rusty-claude-cli/tests/mock_parity_harness.rs` — end-to-end clean-environment harness
+- `scripts/run_mock_parity_harness.sh` — convenience wrapper
+
+## Scenarios
+
+The harness runs these scripted scenarios against a fresh workspace and isolated environment variables:
+
+1. `streaming_text`
+2. `read_file_roundtrip`
+3. `grep_chunk_assembly`
+4. `write_file_allowed`
+5. `write_file_denied`
+
+## Run
+
+```bash
+cd rust/
+./scripts/run_mock_parity_harness.sh
+```
+
+## Manual mock server
+
+```bash
+cd rust/
+cargo run -p mock-anthropic-service -- --bind 127.0.0.1:0
+```
+
+The server prints `MOCK_ANTHROPIC_BASE_URL=...`; point `ANTHROPIC_BASE_URL` at that URL and use any non-empty `ANTHROPIC_API_KEY`.