Docs: consume sphinx-gp-mermaid, drop the vendored renderer

[tony]

Summary

• Replace the vendored docs/_ext/mermaid_inline.py with sphinx-gp-mermaid, the gp-sphinx workspace package it was extracted into — the same build-time dual light/dark inline-SVG pipeline introduced in Docs: build-time mermaid + tmux-layout diagram rendering #1071, now maintained upstream.
• Add the package to the dev and docs dependency groups via a [tool.uv.sources] git source pinned to gp-sphinx's mermaid branch until a release ships.
• Remove the vendored extension, its stylesheet, and its test suite — the code and its tests now live in the gp-sphinx workspace.
• Zero re-render migration: the package kept the cache-key inputs identical, so every existing docs/_mermaid_cache SVG remains a cache hit; adoption never launches Chrome.

Changes

• pyproject.toml / uv.lock: dependency, git source, and exclude-newer-cooldown exemption alongside the other gp-sphinx-family packages.
• docs/conf.py: extra_extensions loads sphinx_gp_mermaid. The explicit myst_fence_as_directive=["mermaid"] stays for now — the pinned gp-sphinx release predates the upstream auto-routing — and drops on the next gp-sphinx bump.
• docs/AGENTS.md, .gitignore, .github/workflows/docs.yml: authoring guidance and comments retargeted at the package. The pnpm/puppeteer provisioning is unchanged: mermaid-cli remains a docs devDependency here, since the Python package resolves mmdc but does not vendor it.

Rendered markup now uses the package's workspace-namespaced CSS classes (gp-sphinx-diagram* in place of gp-diagram*), shipped by the package itself; visual output is unchanged.

Setup required

Hold merge until gp-sphinx cuts a release containing sphinx-gp-mermaid (companion PR below), then flip the git source to a normal PyPI pin. Merging while pointed at a feature branch would break lock reproducibility once that branch is deleted.

Verification

Zero references to the vendored renderer or its CSS namespace remain:

rg -l 'mermaid_inline|gp-diagram'

Test plan

• uv run ruff check ., uv run ruff format ., uv run mypy . — clean
• uv run py.test --reruns 0 — suite green (the vendored mermaid tests moved upstream with the code)
• just build-docs — every diagram page renders through the package; _mermaid_cache entries hit with no mmdc invocations (mtimes unchanged)
• Built pages carry gp-sphinx-diagram__variant--theme-{light,dark} figures and link the packaged stylesheet

Companion PR

• sphinx-gp-mermaid: build-time Mermaid diagrams as dual inline SVG git-pull/gp-sphinx#61 — the extracted sphinx-gp-mermaid package

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 82.56%. Comparing base (085f9d9) to head (533e6a5).

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #1073   +/-   ##
=======================================
  Coverage   82.56%   82.56%           
=======================================
  Files          31       31           
  Lines        2770     2770           
  Branches      518      518           
=======================================
  Hits         2287     2287           
  Misses        346      346           
  Partials      137      137           

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