docs: GUI walkthrough installs from the lineforge repo

[RFingAdam]

examples/08_gui_walkthrough.md told readers to git clone https://github.com/RFingAdam/lineforge-gui and set up a separate backend venv and frontend there. That repo does not exist (the GUI was merged into lineforge itself, under frontend/ and src/lineforge/web/), so the instructions fail today and would fetch whatever code someone later puts at that name.

Changes:

• Intro: the GUI ships inside the lineforge repo (lineforge.web backend, frontend/ Next.js app).
• Prerequisites: clone RFingAdam/lineforge and pip install -e ".[gui]" (with the Rust toolchain note from the README), plus Node and pnpm on PATH. The separate backend venv and manual pnpm install steps are gone because lineforge gui handles the frontend deps.
• Launch steps: match what lineforge gui in src/lineforge/cli.py does now (checks frontend/, installs node_modules if missing, runs uvicorn lineforge.web.app:app and pnpm dev, moves to a free port if busy).
• The walkthrough, stopping and no-chat sections are unchanged.

Checked:

• Re-grepped the repo for lineforge-gui: the only hit left is the private npm package name lineforge-gui-frontend in frontend/package.json, which is not a URL.
• Steps checked against the gui command in src/lineforge/cli.py, the gui extra in pyproject.toml, and the dev/start scripts in frontend/package.json. Tool names in the walkthrough (set_geometry_type, set_geometry_field, target_z0) still exist in src/lineforge/web/agent.py.
• Only examples/08_gui_walkthrough.md is touched.