list_files with_metadata mode, deeper JSON outline, MCP connector descriptions in prompt

- list_files: new with_metadata parameter returns {path, line_count?, size}
  per entry (both disk and memory-docs) so the agent can spot large files
  worth outlining before reading
- ast_outline: replaced flat tree-sitter JSON walker with a recursive one
  that shows nested keys at every depth, with inline scalar values and
  container summaries
- message_builder: format active MCP connectors as a table with description
  instead of a bare bullet list
- read_file description now hints to use get_ast_outline first
- tools.md: agent guidance to outline before reading

crates/skald-core/src/session/handler/message_builder.rs

@@ -618,9 +618,13 @@ impl MessageBuilder {
         }
 
         if !active.is_empty() {
-            out.push_str("\n**Active** — tools callable as `mcp__<name>__<tool>`:\n");
+            out.push_str("\n**Active** — tools callable as `mcp__<name>__<tool>`:\n\n");
+            out.push_str("| Server | Description |\n|--------|-------------|\n");
             for name in &active {
-                out.push_str(&format!("- `{name}`\n"));
+                let desc = descriptions.get(*name)
+                    .and_then(|d| d.as_deref())
+                    .unwrap_or("—");
+                out.push_str(&format!("| `{name}` | {desc} |\n"));
             }
         }