docs: llms.txt, llms-full.txt and AGENTS.md

[petercorke]

Follows the pattern of ansitable and bdsim.

llms.txt and llms-full.txt

• docs/llms_intro.md: hand-written, the part of llms.txt that an assistant that is using pgraph needs: what it is, a quick start, key points, common mistakes. Every claim in it was checked by running code (the quick start prints ['a', 'b', 'c'] 12.0; the planners return None for no path; component(c) takes a number; an in-place coordinate edit is not seen by the k-d tree; and so on).
• docs/llms_txt.py: ansitable's generator, with one change: pgraph's toctree is nested (index → api → common → sections), so it follows toctrees recursively. llms.txt is the introduction followed by the list of pages; llms-full.txt is the Sphinx site rendered by the text builder, API reference included (13 pages, 175 kB, no raw directives or tracebacks).
• docs/Makefile has a llms target, and the docs job of ci.yml runs make -C docs html llms, so both files are published with the site. Tried locally with exactly that command. The docs job runs only on pushes to main, so the files appear on the site after this merges.

AGENTS.md

Points assistants that are using pgraph to llms.txt, and records what an agent working on the source needs: the layout and the one Vertex class, the records that must agree (validate()), the two staleness flags and why they are separate, why Vertex.__setattr__ is hot and what writes __dict__ directly, pickling by a flat description, how tests and the scanners (Codacy, Codecov, mypy) behave, that performance claims are measured against 0.6.5, how a release is made, and that scipy must stay out of pyproject.toml. llms.txt links back to it under "Optional".

Also: a pointer to both files in the README.

🤖 Generated with Claude Code

[codacy-production]

Up to standards ✅

🟢 Issues 0 issues

Results:
0 new issues

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🟢 Metrics 26 complexity · 0 duplication

Metric	Results
Complexity	26
Duplication	0

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 97.26%. Comparing base (8210675) to head (0311897).
⚠️ Report is 2 commits behind head on main.

Additional details and impacted files

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##             main      #68   +/-   ##
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  Coverage   97.26%   97.26%           
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  Files           3        3           
  Lines        1570     1570           
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  Hits         1527     1527           
  Misses         43       43           

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