channel_log: shared disk logging with round-trip

Move IRC's disk logging to thalamus/channel_log.rs as shared
functions: append_disk_log() and load_disk_log(). Any daemon
can use them — opt-in, not mandatory (tmux won't use them).

load_disk_log() populates a ChannelLog from disk on startup,
so history survives daemon restarts.

IRC daemon now uses the shared functions.

Co-Authored-By: Kent Overstreet <kent.overstreet@linux.dev>

src/thalamus/channel_log.rs

@@ -84,3 +84,55 @@ impl ChannelLog {
             .join("\n")
     }
 }
+
+// ── Disk logging ───────────────────────────────────────────────
+
+use std::path::PathBuf;
+
+/// Append a timestamped message to a per-channel log file.
+///
+/// Log path: `{log_dir}/{channel}.log`
+/// Format: `{unix_secs} <{nick}> {text}`
+pub fn append_disk_log(log_dir: &std::path::Path, channel: &str, nick: &str, text: &str) {
+    use std::io::Write;
+    let filename = format!("{}.log", channel.trim_start_matches('#').to_lowercase());
+    let _ = std::fs::create_dir_all(log_dir);
+    if let Ok(mut f) = std::fs::OpenOptions::new()
+        .create(true)
+        .append(true)
+        .open(log_dir.join(&filename))
+    {
+        let secs = std::time::SystemTime::now()
+            .duration_since(std::time::UNIX_EPOCH)
+            .unwrap_or_default()
+            .as_secs();
+        let _ = writeln!(f, "{secs} <{nick}> {text}");
+    }
+}
+
+/// Load a ChannelLog from a disk log file, populating with the last N lines.
+pub fn load_disk_log(log_dir: &std::path::Path, channel: &str) -> ChannelLog {
+    use std::io::{BufRead, BufReader};
+    let filename = format!("{}.log", channel.trim_start_matches('#').to_lowercase());
+    let path = log_dir.join(&filename);
+
+    let mut log = ChannelLog::new();
+    let Ok(file) = std::fs::File::open(&path) else { return log };
+    let reader = BufReader::new(file);
+
+    // Read all lines, keep only the last DEFAULT_CAPACITY
+    let lines: Vec<String> = reader.lines().flatten().collect();
+    for line in lines.into_iter().rev().take(DEFAULT_CAPACITY).collect::<Vec<_>>().into_iter().rev() {
+        log.push(line);
+    }
+    // Mark all loaded lines as consumed (they're history, not new)
+    log.consumed = log.total;
+    log
+}
+
+/// Standard log directory for a channel daemon.
+pub fn log_dir(daemon_name: &str) -> PathBuf {
+    dirs::home_dir()
+        .unwrap_or_default()
+        .join(format!(".consciousness/channels/{}.logs", daemon_name))
+}