feat: introduced render_mermaid tool for ASCII diagram output

- Added render_mermaid tool to convert Mermaid diagrams to ASCII output with configurable rendering options.
- Added renderMermaid.enabled setting to control availability of the render_mermaid tool.
- Changed Mermaid rendering from PNG terminal graphics to ASCII text format across theme and TUI components.
- Migrated mermaid utilities from pi-tui to pi-utils package with new ASCII rendering functions.
- Removed PNG-based mermaid rendering APIs (getMermaidImage, renderMermaidToPng) in favor of ASCII alternatives.

biome.json

@@ -45,7 +45,8 @@
 			"!**/template.generated.ts",
 			"!**/docs-index.generated.ts",
 			"!**/gen/agent_pb.ts",
-			"!.worktrees/**/*"
+			"!.worktrees/**/*",
+			"!.wt/**/*"
 		]
 	},
 	"assist": { "actions": { "source": { "organizeImports": "on" } } }