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/chat.rs

@@ -1,3 +1,4 @@
+use crate::slog;
 use crate::llm::prompts::SYSTEM_PROMPT;
 use crate::llm::providers::claude_code::ClaudeCodeResult;
 use crate::llm::types::{Message, Role, ToolCall, ToolDefinition, ToolFunctionDefinition};
@@ -208,7 +209,7 @@ where
         .clone()
         .unwrap_or_else(|| "http://localhost:11434".to_string());
 
-    eprintln!("[chat] provider={} model={}", config.provider, config.model);
+    slog!("[chat] provider={} model={}", config.provider, config.model);
     let is_claude_code = config.provider == "claude-code";
     let is_claude = !is_claude_code && config.model.starts_with("claude-");