docs: install from GitHub, not PyPI

[RFingAdam]

Neither lineforge nor atlc3 is published on PyPI, but the docs told people to pip install them by name. Anyone could register those names and ship code to readers who follow the docs (raised in #16).

This points every install at the GitHub repo and says plainly that a package under either name on PyPI is not this project.

• README, docs/quickstart.md, examples/03_mcp_claude_desktop.md: pip install "lineforge @ git+https://github.com/RFingAdam/lineforge.git", or a clone plus pip install -e for development and the GUI. Removes the claim that PyPI wheels exist.
• MIGRATION.md and README: atlc3 was never on PyPI. Pin it by tag (atlc3 @ git+…@v1.1.0) instead of atlc3==1.1.0.
• lineforge gui missing-extras hint and the [gui] comment in pyproject.toml: install the extras from a clone.

Release-planning docs (docs/plan.md, AUDIT.md, docs/release/) are unchanged; they describe a future release, not current availability.

Checked: both documented commands in fresh Python 3.12 venvs. The lineforge install builds the Rust core and solves the quickstart microstrip to Z0 = 51.93 Ω. The atlc3 tag pin installs 1.1.0. The new CLI hint renders through Rich as pip install -e '.[gui]'. The docs mkdocs build --strict runs on this PR.

[RFingAdam]

Docs CI can't pick this up: docs.yml needs a [self-hosted, Linux, X64, haven] runner and this repo has none registered, so the job stays queued (every Docs run on main since 2026-09-10 was cancelled the same way). Ran the same mkdocs build --strict locally on this branch with the docs extras and lineforge installed: exit 0, no warnings, and the rendered quick start shows the GitHub install.