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/main.rs

@@ -3,6 +3,7 @@ mod config;
 mod http;
 mod io;
 mod llm;
+pub mod log_buffer;
 mod state;
 mod store;
 mod workflow;
@@ -39,7 +40,7 @@ async fn main() -> Result<(), std::io::Error> {
         )
         .await
         .unwrap_or_else(|e| {
-            eprintln!("Warning: failed to auto-open project at {project_root:?}: {e}");
+            slog!("Warning: failed to auto-open project at {project_root:?}: {e}");
             project_root.to_string_lossy().to_string()
         });
 
@@ -82,7 +83,7 @@ async fn main() -> Result<(), std::io::Error> {
     // active pipeline stages (2_current/, 3_qa/, 4_merge/).
     if let Some(root) = startup_root {
         tokio::spawn(async move {
-            eprintln!("[auto-assign] Server startup: scanning pipeline stages for unassigned work.");
+            slog!("[auto-assign] Server startup: scanning pipeline stages for unassigned work.");
             startup_agents.auto_assign_available_work(&root).await;
         });
     }