feat: add web UI prompt library from canonical workflows (#428)

Surface short copy/paste operator prompts derived from canonical workflow
files with workflow path and SHA-256 citations. Adds /prompts, /prompts/{id},
and /api/prompts with one-click copy.

Closes #428

webui/app.py

@@ -12,8 +12,8 @@ from starlette.routing import Route
 from webui.layout import render_page
 from webui.project_registry import find_project, load_registry, registry_to_dict
 from webui.project_views import render_project_detail, render_projects_list
-from webui.prompt_library import library_to_dict
-from webui.prompt_views import render_prompts_page
+from webui.prompt_library import find_prompt, library_to_dict
+from webui.prompt_views import render_prompt_detail, render_prompts_page
 
 _READ_ONLY_METHODS = frozenset({"GET", "HEAD", "OPTIONS"})
 
@@ -85,6 +85,24 @@ async def prompts(_request: Request) -> HTMLResponse:
     return HTMLResponse(render_prompts_page())
 
 
+async def prompt_detail(request: Request) -> HTMLResponse:
+    prompt_id = request.path_params["prompt_id"]
+    prompt = find_prompt(prompt_id)
+    if prompt is None:
+        return HTMLResponse(
+            render_page(
+                title="Prompt not found",
+                body_html=(
+                    "<h2>Prompt not found</h2>"
+                    f"<p>No library entry for <code>{prompt_id}</code>.</p>"
+                    '<p><a href="/prompts">← All prompts</a></p>'
+                ),
+            ),
+            status_code=404,
+        )
+    return HTMLResponse(render_prompt_detail(prompt))
+
+
 async def api_prompts(_request: Request) -> JSONResponse:
     return JSONResponse(library_to_dict())
 
@@ -137,6 +155,7 @@ def create_app() -> Starlette:
             Route("/projects/{project_id}", project_detail, methods=["GET"]),
             Route("/api/projects", api_projects, methods=["GET"]),
             Route("/prompts", prompts, methods=["GET"]),
+            Route("/prompts/{prompt_id}", prompt_detail, methods=["GET"]),
             Route("/api/prompts", api_prompts, methods=["GET"]),
             Route("/runtime", runtime, methods=["GET"]),
             Route("/audit", audit, methods=["GET"]),