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/io/search.rs

@@ -1,3 +1,4 @@
+use crate::slog;
 use crate::state::SessionState;
 use ignore::WalkBuilder;
 use serde::Serialize;
@@ -52,7 +53,7 @@ pub async fn search_files_impl(query: String, root: PathBuf) -> Result<Vec<Searc
                         });
                     }
                 }
-                Err(err) => eprintln!("Error walking dir: {}", err),
+                Err(err) => slog!("Error walking dir: {}", err),
             }
         }