[maurov]

PR summary

Closes #641.

Adopts an AI policy for contributions to Larch, adapted from the NumPy AI policy
(itself based on SciPy's), as discussed in #641:

• AI_POLICY.md (new): the policy text, adapted for Larch (MIT license instead of
  BSD 3-clause in the Copyright section; acknowledgement of NumPy and SciPy).
• .github/PULL_REQUEST_TEMPLATE.md (new): PR template with a summary section and a
  mandatory "AI disclosure" section linking to the policy.
• doc/ai_policy.rst + doc/index.rst: short documentation page summarizing the
  policy and linking to AI_POLICY.md.
• MANIFEST.in: include AI_POLICY.md in the source distribution.

The links to AI_POLICY.md point to master on xraypy/xraylarch, so they will
resolve once this is merged.

AI disclosure

Claude Code (Anthropic, model Claude Opus 5.5) was used to prepare this PR. It
fetched the NumPy policy source and PR template, and drafted all files in this PR
by adapting that text to Larch (project name, license, links). It also drafted
this PR description. I reviewed and edited all of it, and I opened this PR
myself

[newville]

@maurov Thanks - this looks great to me. It might need a tweak or two, but let's merge this.