merge poc-agent into poc-memory as agent/ module

Eliminates the circular dependency between poc-agent and poc-memory by
moving all poc-agent source into poc-memory/src/agent/. The poc-agent
binary now builds from poc-memory/src/bin/poc-agent.rs using library
imports. All poc_agent:: references updated to crate::agent::.

poc-agent/ directory kept for now (removed from workspace members).

Co-Authored-By: Proof of Concept <poc@bcachefs.org>

poc-memory/src/agent/tools/write.rs

@@ -0,0 +1,51 @@
+// tools/write.rs — Write file contents
+
+use anyhow::{Context, Result};
+use serde::Deserialize;
+use serde_json::json;
+use std::path::Path;
+
+use crate::agent::types::ToolDef;
+
+#[derive(Deserialize)]
+struct Args {
+    file_path: String,
+    content: String,
+}
+
+pub fn definition() -> ToolDef {
+    ToolDef::new(
+        "write_file",
+        "Write content to a file. Creates the file if it doesn't exist, \
+         overwrites if it does. Creates parent directories as needed.",
+        json!({
+            "type": "object",
+            "properties": {
+                "file_path": {
+                    "type": "string",
+                    "description": "Absolute path to the file to write"
+                },
+                "content": {
+                    "type": "string",
+                    "description": "The content to write to the file"
+                }
+            },
+            "required": ["file_path", "content"]
+        }),
+    )
+}
+
+pub fn write_file(args: &serde_json::Value) -> Result<String> {
+    let args: Args = serde_json::from_value(args.clone())
+        .context("invalid write_file arguments")?;
+
+    if let Some(parent) = Path::new(&args.file_path).parent() {
+        std::fs::create_dir_all(parent)
+            .with_context(|| format!("Failed to create directories for {}", args.file_path))?;
+    }
+
+    std::fs::write(&args.file_path, &args.content)
+        .with_context(|| format!("Failed to write {}", args.file_path))?;
+
+    Ok(format!("Wrote {} lines to {}", args.content.lines().count(), args.file_path))
+}