Replace sphinx-llm with Yardang builder

[timkpaine]

Description

Replace sphinx-llm with a lightweight Yardang Sphinx builder that generates llms.txt, optional llms-full.txt, and linked Markdown pages in toctree order.

yardang build now runs HTML and LLM builders explicitly with shared configuration. This removes the nested subprocess workaround and unused parallel/suffix settings while preserving [tool.yardang.llms] configuration. Orphan pages are excluded, and page descriptions come from metadata or document content.

Type of Change

• Bug fix
• New feature
• Documentation update
• Refactor / code cleanup
• CI / build configuration
• Other (describe below)

Checklist

• Linting passes (make lint)
• Tests pass (make test)
• New tests added for new functionality
• Documentation updated (if applicable)
• Changelog / version bump (if applicable)

[github-actions]

Test Results

83 tests  ±0   83 ✅ ±0   32s ⏱️ +14s
 1 suites ±0    0 💤 ±0 
 1 files   ±0    0 ❌ ±0 

Results for commit 5080ad1. ± Comparison against base commit d46c41f.

This pull request removes 4 and adds 4 tests. Note that renamed tests count towards both.

yardang.tests.test_llms.TestLlmsConfiguration ‑ test_generate_docs_llms_disabled_by_default
yardang.tests.test_llms.TestLlmsConfiguration ‑ test_generate_docs_with_llms_enabled
yardang.tests.test_llms.TestLlmsConfiguration ‑ test_llms_config_defaults
yardang.tests.test_llms.TestLlmsConfiguration ‑ test_llms_config_loading_from_pyproject

yardang.tests.test_llms ‑ test_build_generates_llms_outputs
yardang.tests.test_llms ‑ test_full_build_can_be_disabled
yardang.tests.test_llms ‑ test_generated_configuration_enables_yardang_llms
yardang.tests.test_llms ‑ test_llms_generation_is_disabled_by_default

[codecov]

Codecov Report

❌ Patch coverage is 50.66667% with 74 lines in your changes missing coverage. Please review.
✅ Project coverage is 83.48%. Comparing base (27c3335) to head (5080ad1).
⚠️ Report is 3 commits behind head on main.

Files with missing lines	Patch %	Lines
yardang/sphinx/llms.py	0.00%	73 Missing ⚠️
yardang/cli.py	94.44%	0 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #134      +/-   ##
==========================================
- Coverage   87.05%   83.48%   -3.57%     
==========================================
  Files          14       15       +1     
  Lines        1761     1823      +62     
  Branches      183      197      +14     
==========================================
- Hits         1533     1522      -11     
- Misses        177      250      +73     
  Partials       51       51              

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