Move poc-agent into workspace, improve agent prompts

Move poc-agent (substrate-independent AI agent framework) into the
memory workspace as a step toward using its API client for direct
LLM calls instead of shelling out to claude CLI.

Agent prompt improvements:
- distill: rewrite from hub-focused to knowledge-flow-focused.
  Now walks upward from seed nodes to find and refine topic nodes,
  instead of only maintaining high-degree hubs.
- distill: remove "don't touch journal entries" restriction
- memory-instructions-core: add "Make it alive" section — write
  with creativity and emotional texture, not spreadsheet summaries
- memory-instructions-core: add "Show your reasoning" section —
  agents must explain decisions, especially when they do nothing
- linker: already had emotional texture guidance (kept as-is)

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

poc-agent/src/tools/edit.rs

@@ -0,0 +1,92 @@
+// 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_json::json;
+
+use crate::types::ToolDef;
+
+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 path = args["file_path"]
+        .as_str()
+        .context("file_path is required")?;
+    let old_string = args["old_string"]
+        .as_str()
+        .context("old_string is required")?;
+    let new_string = args["new_string"]
+        .as_str()
+        .context("new_string is required")?;
+    let replace_all = args["replace_all"].as_bool().unwrap_or(false);
+
+    if old_string == new_string {
+        anyhow::bail!("old_string and new_string are identical");
+    }
+
+    let content =
+        std::fs::read_to_string(path).with_context(|| format!("Failed to read {}", path))?;
+
+    if replace_all {
+        let count = content.matches(old_string).count();
+        if count == 0 {
+            anyhow::bail!("old_string not found in {}", path);
+        }
+        let new_content = content.replace(old_string, new_string);
+        std::fs::write(path, &new_content)
+            .with_context(|| format!("Failed to write {}", path))?;
+        Ok(format!("Replaced {} occurrences in {}", count, path))
+    } else {
+        let count = content.matches(old_string).count();
+        if count == 0 {
+            anyhow::bail!("old_string not found in {}", path);
+        }
+        if count > 1 {
+            anyhow::bail!(
+                "old_string appears {} times in {} — use replace_all or provide more context \
+                 to make it unique",
+                count,
+                path
+            );
+        }
+        let new_content = content.replacen(old_string, new_string, 1);
+        std::fs::write(path, &new_content)
+            .with_context(|| format!("Failed to write {}", path))?;
+        Ok(format!("Edited {}", path))
+    }
+}