M1: tool trait, permission service, config, Provider trait, engine loop

Adds the Tool trait, config loading/schema, the Provider trait plus an Llm event surface, a permission service, and the core engine loop with a doom-loop guard and retry scaffolding. The processor drives a text/tool-call/final-text turn against a Provider, laying the groundwork for the MockProvider integration test and the real Anthropic path.

crates/harness-core/src/engine/system.rs

@@ -0,0 +1,56 @@
+use std::path::Path;
+
+/// cwd, platform, date, best-effort git status. Kept as its own block (not merged into the
+/// agent prompt) because Anthropic cache breakpoints need the system prompt structured as
+/// separate blocks (see `LlmRequest.system: Vec<String>`).
+pub fn env_header(cwd: &Path) -> String {
+    let git_status = std::process::Command::new("git")
+        .arg("status")
+        .arg("--short")
+        .current_dir(cwd)
+        .output()
+        .ok()
+        .filter(|o| o.status.success())
+        .map(|o| String::from_utf8_lossy(&o.stdout).trim().to_string())
+        .filter(|s| !s.is_empty());
+
+    let mut header = format!(
+        "Working directory: {}\nPlatform: {}\n",
+        cwd.display(),
+        std::env::consts::OS
+    );
+    if let Some(status) = git_status {
+        header.push_str(&format!("Git status:\n{status}\n"));
+    }
+    header
+}
+
+/// Ordered system prompt blocks: environment header, agent prompt, then project
+/// instructions (e.g. AGENTS.md contents). Order matches `02-engine.md`.
+pub fn assemble(env_header: String, agent_prompt: &str, instructions: &[String]) -> Vec<String> {
+    let mut blocks = vec![env_header, agent_prompt.to_string()];
+    blocks.extend(instructions.iter().cloned());
+    blocks
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    #[test]
+    fn env_header_includes_cwd_and_platform() {
+        let header = env_header(Path::new("."));
+        assert!(header.contains("Working directory:"));
+        assert!(header.contains(std::env::consts::OS));
+    }
+
+    #[test]
+    fn assemble_orders_env_agent_then_instructions() {
+        let blocks = assemble(
+            "ENV".to_string(),
+            "AGENT",
+            &["AGENTS.md contents".to_string()],
+        );
+        assert_eq!(blocks, vec!["ENV", "AGENT", "AGENTS.md contents"]);
+    }
+}