chore: install Claude scaffold

Add CLAUDE.md with project-specific rules (test structure, naming
conventions, MCP tools reference). Add .mcp.json for rust-mcp and
crates-io MCP servers. Add rust-toolchain.toml pinning stable with
rustfmt and clippy. Add .claude/settings.json with PostToolUse hook
that runs cargo test automatically after every file edit.

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

.claude/settings.json

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+{
+  "hooks": {
+    "PostToolUse": [
+      {
+        "matcher": "Write|Edit",
+        "type": "command",
+        "command": "bash -c 'if [ -f Cargo.toml ]; then if command -v cargo-nextest >/dev/null 2>&1; then cargo nextest run 2>&1; else cargo test 2>&1; fi; fi'",
+        "timeout": 120
+      }
+    ]
+  }
+}

.mcp.json

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+{
+  "mcpServers": {
+    "rust-mcp": {
+      "type": "stdio",
+      "command": "rust-mcp-server",
+      "args": []
+    },
+    "crates": {
+      "type": "stdio",
+      "command": "crates-mcp",
+      "args": []
+    }
+  }
+}

CLAUDE.md

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+# Rust Project — Claude Instructions
+
+## Mandatory Rules
+
+1. **Always write tests** alongside production code — no feature ships without tests
+2. **Always verify tests pass** after every change — the PostToolUse hook runs automatically;
+   if it shows failures, fix them before moving on
+3. Run `cargo clippy -- -D warnings` and resolve all warnings
+4. Run `cargo fmt` before considering any task complete
+
+## Available MCP Tools
+
+Install with `curl -sSf https://raw.githubusercontent.com/USUARIO/claude-rust-scaffold/main/install.sh | sh`
+
+| Server | Tools | Purpose |
+|--------|-------|---------|
+| `rust-mcp` | `cargo_check`, `cargo_build`, `cargo_test`, `cargo_clippy`, `cargo_fmt`, `cargo_add` | Run cargo commands directly |
+| `crates` | search, versions, dependencies, docs | Explore crates.io and docs.rs |
+
+## Test Structure
+
+### Unit Tests — inside `src/`
+
+Place at the bottom of each source file:
+
+```rust
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    #[test]
+    fn should_return_error_when_input_is_empty() {
+        // arrange
+        let input = "";
+        // act
+        let result = parse(input);
+        // assert
+        assert!(result.is_err());
+    }
+}
+```
+
+- Name tests descriptively: `should_<outcome>_when_<condition>`
+- Cover: happy path, edge cases (empty, max values), error cases
+
+### Integration Tests — `tests/` directory
+
+- One file per feature or behavior
+- Use only public interfaces (`pub`)
+- Simulate real usage end-to-end
+
+```rust
+// tests/parsing.rs
+use tmuxido::parse;
+
+#[test]
+fn parses_valid_input_successfully() {
+    let result = parse("valid input");
+    assert!(result.is_ok());
+}
+```
+
+### Snapshot Testing with `insta`
+
+For complex outputs or large structs:
+
+```rust
+#[test]
+fn renders_report_correctly() {
+    let report = generate_report(&data);
+    insta::assert_snapshot!(report);
+}
+```
+
+Review snapshots: `cargo insta review`
+
+### Property Testing with `proptest`
+
+For pure functions over wide input domains:
+
+```rust
+use proptest::prelude::*;
+
+proptest! {
+    #[test]
+    fn round_trip_encode_decode(s in ".*") {
+        let encoded = encode(&s);
+        prop_assert_eq!(decode(&encoded), s);
+    }
+}
+```
+
+## Recommended `Cargo.toml` dev-dependencies
+
+```toml
+[dev-dependencies]
+proptest = "1"
+insta = { version = "1", features = ["json", "yaml"] }
+mockall = "0.13"
+
+# if async:
+tokio = { version = "1", features = ["full", "test-util"] }
+```
+
+## Recommended Project Structure
+
+```
+tmuxido/
+├── Cargo.toml
+├── src/
+│   ├── lib.rs          # core logic (unit tests at bottom)
+│   ├── main.rs         # entrypoint (thin, delegates to lib)
+│   └── module/
+│       └── mod.rs      # #[cfg(test)] mod tests {} at bottom
+├── tests/
+│   └── integration.rs  # integration tests
+└── benches/
+    └── bench.rs        # benchmarks (optional)
+```
+
+Prefer `lib.rs` + `main.rs` split so logic stays testable independently of the binary entrypoint.

rust-toolchain.toml

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+[toolchain]
+channel = "stable"
+components = ["rustfmt", "clippy"]