feat(story-93): expose server logs to agents via get_server_logs MCP tool

- Add log_buffer module: bounded 1000-line ring buffer with push/get_recent API
- Add slog! macro: drop-in for eprintln! that also captures to ring buffer
- Replace all eprintln! calls across agents, watcher, search, chat, worktree, claude_code with slog!
- Add get_server_logs MCP tool: accepts count (1-500) and optional filter params
- 5 unit tests for log_buffer covering push/retrieve, eviction, filtering, count limits, empty buffer
- 262 tests passing, clippy clean

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

server/src/llm/providers/claude_code.rs

@@ -1,3 +1,4 @@
+use crate::slog;
 use portable_pty::{CommandBuilder, PtySize, native_pty_system};
 use std::io::{BufRead, BufReader, Write};
 use std::sync::Arc;
@@ -152,7 +153,7 @@ fn run_pty_session(
     // Allow nested spawning when the server itself runs inside Claude Code
     cmd.env("CLAUDECODE", "");
 
-    eprintln!(
+    slog!(
         "[pty-debug] Spawning: claude -p \"{}\" {} --output-format stream-json --verbose",
         user_message,
         resume_session_id
@@ -165,7 +166,7 @@ fn run_pty_session(
         .spawn_command(cmd)
         .map_err(|e| format!("Failed to spawn claude: {e}"))?;
 
-    eprintln!(
+    slog!(
         "[pty-debug] Process spawned, pid: {:?}",
         child.process_id()
     );
@@ -187,23 +188,23 @@ fn run_pty_session(
 
     std::thread::spawn(move || {
         let buf_reader = BufReader::new(reader);
-        eprintln!("[pty-debug] Reader thread started");
+        slog!("[pty-debug] Reader thread started");
         for line in buf_reader.lines() {
             match line {
                 Ok(l) => {
-                    eprintln!("[pty-debug] raw line: {}", l);
+                    slog!("[pty-debug] raw line: {}", l);
                     if line_tx.send(Some(l)).is_err() {
                         break;
                     }
                 }
                 Err(e) => {
-                    eprintln!("[pty-debug] read error: {e}");
+                    slog!("[pty-debug] read error: {e}");
                     let _ = line_tx.send(None);
                     break;
                 }
             }
         }
-        eprintln!("[pty-debug] Reader thread done");
+        slog!("[pty-debug] Reader thread done");
         let _ = line_tx.send(None);
     });
 
@@ -223,7 +224,7 @@ fn run_pty_session(
                     continue;
                 }
 
-                eprintln!(
+                slog!(
                     "[pty-debug] processing: {}...",
                     &trimmed[..trimmed.len().min(120)]
                 );