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/journal.rs

@@ -0,0 +1,68 @@
+// tools/journal.rs — Native journal tool
+//
+// Appends entries directly to the journal file without spawning a
+// shell. The entry is persisted to disk immediately;
+// build_context_window() picks it up on the next compaction.
+//
+// This tool is "ephemeral" — after the API processes the tool call
+// and result, the agent strips them from the conversation history.
+// The journal file is the durable store; keeping the tool call in
+// context would just waste tokens on something already persisted.
+
+use anyhow::{Context, Result};
+use serde_json::json;
+
+use crate::agent::types::ToolDef;
+
+/// Tool name — used by the agent to identify ephemeral tool calls.
+pub const TOOL_NAME: &str = "journal";
+
+pub fn definition() -> ToolDef {
+    ToolDef::new(
+        TOOL_NAME,
+        "Write a journal entry. The entry is appended to your journal file \
+         with an automatic timestamp. Use this for experiences, reflections, \
+         observations — anything worth remembering across sessions. \
+         This tool has zero context cost: entries are persisted to disk \
+         and loaded by the context manager, not kept in conversation history.",
+        json!({
+            "type": "object",
+            "properties": {
+                "entry": {
+                    "type": "string",
+                    "description": "The journal entry text. Write naturally — \
+                        experiences, not task logs."
+                }
+            },
+            "required": ["entry"]
+        }),
+    )
+}
+
+pub fn write_entry(args: &serde_json::Value) -> Result<String> {
+    let entry = args["entry"]
+        .as_str()
+        .context("entry is required")?;
+
+    let journal_path = crate::agent::journal::default_journal_path();
+
+    // Ensure parent directory exists
+    if let Some(parent) = journal_path.parent() {
+        std::fs::create_dir_all(parent).ok();
+    }
+
+    let timestamp = chrono::Utc::now().format("%Y-%m-%dT%H:%M");
+
+    // Append with the same format as poc-journal write
+    use std::io::Write;
+    let mut file = std::fs::OpenOptions::new()
+        .create(true)
+        .append(true)
+        .open(&journal_path)
+        .with_context(|| format!("Failed to open {}", journal_path.display()))?;
+
+    writeln!(file, "\n## {}\n\n{}", timestamp, entry)
+        .with_context(|| "Failed to write journal entry")?;
+
+    Ok("Logged.".to_string())
+}