v0.9.14: model-facing open/reveal tool — open_in_editor(path, line) / open_url through the app-server

Source: tool-surface survey 2026-09-15. Codewhale has an internal `open_url()` helper (`utils.rs`) but no model-callable tool.

With GPUI now the product client, `open_in_editor(path, line)` / `open_url(url)` lets the agent steer the user's attention to the exact code it just changed — cheap, and it makes the desktop client feel native rather than a terminal emulation.

## Proposal

A model-facing `open`/`reveal` tool surfaced through the app-server to the connected client (GPUI first; TUI can fall back to the existing `open_url` helper for URLs). Needs a client capability bit so the tool is only advertised when a surface exists.

Start by reading the existing open_url() helper in utils.rs and tracing how tools cross the app-server to connected clients. Define the client capability advertisement and implement the model-facing open/reveal flow for GPUI, with the stated TUI URL fallback; done when surfaces without the capability do not advertise the tool.

Written by the indexing model from the issue text.