flatten: move poc-memory contents to workspace root

No more subcrate nesting — src/, agents/, schema/, defaults/, build.rs
all live at the workspace root. poc-daemon remains as the only workspace
member. Crate name (poc-memory) and all imports unchanged.

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

src/agent/tools/edit.rs

@@ -0,0 +1,90 @@
+// tools/edit.rs — Search-and-replace file editing
+//
+// The edit tool performs exact string replacement in files. This is the
+// same pattern used by Claude Code and aider — it's more reliable than
+// line-number-based editing because the model specifies what it sees,
+// not where it thinks it is.
+//
+// Supports replace_all for bulk renaming (e.g. variable renames).
+
+use anyhow::{Context, Result};
+use serde::Deserialize;
+use serde_json::json;
+
+use crate::agent::types::ToolDef;
+
+#[derive(Deserialize)]
+struct Args {
+    file_path: String,
+    old_string: String,
+    new_string: String,
+    #[serde(default)]
+    replace_all: bool,
+}
+
+pub fn definition() -> ToolDef {
+    ToolDef::new(
+        "edit_file",
+        "Perform exact string replacement in a file. The old_string must appear \
+         exactly once in the file (unless replace_all is true). Use read_file first \
+         to see the current contents.",
+        json!({
+            "type": "object",
+            "properties": {
+                "file_path": {
+                    "type": "string",
+                    "description": "Absolute path to the file to edit"
+                },
+                "old_string": {
+                    "type": "string",
+                    "description": "The exact text to find and replace"
+                },
+                "new_string": {
+                    "type": "string",
+                    "description": "The replacement text"
+                },
+                "replace_all": {
+                    "type": "boolean",
+                    "description": "Replace all occurrences (default false)"
+                }
+            },
+            "required": ["file_path", "old_string", "new_string"]
+        }),
+    )
+}
+
+pub fn edit_file(args: &serde_json::Value) -> Result<String> {
+    let a: Args = serde_json::from_value(args.clone())
+        .context("invalid edit_file arguments")?;
+
+    if a.old_string == a.new_string {
+        anyhow::bail!("old_string and new_string are identical");
+    }
+
+    let content = std::fs::read_to_string(&a.file_path)
+        .with_context(|| format!("Failed to read {}", a.file_path))?;
+
+    let count = content.matches(&*a.old_string).count();
+    if count == 0 {
+        anyhow::bail!("old_string not found in {}", a.file_path);
+    }
+
+    if a.replace_all {
+        let new_content = content.replace(&*a.old_string, &a.new_string);
+        std::fs::write(&a.file_path, &new_content)
+            .with_context(|| format!("Failed to write {}", a.file_path))?;
+        Ok(format!("Replaced {} occurrences in {}", count, a.file_path))
+    } else {
+        if count > 1 {
+            anyhow::bail!(
+                "old_string appears {} times in {} — use replace_all or provide more context \
+                 to make it unique",
+                count, a.file_path
+            );
+        }
+        let new_content = content.replacen(&*a.old_string, &a.new_string, 1);
+        std::fs::write(&a.file_path, &new_content)
+            .with_context(|| format!("Failed to write {}", a.file_path))?;
+        Ok(format!("Edited {}", a.file_path))
+    }
+}