Point README AI-review link to xarray-spatial-skills repo

[brendancol]

Closes #3668

The README's top callout linked the AI review workflow to .claude/commands/, which is gitignored (.gitignore:107) and never committed, so the link 404s on GitHub. The command suite now lives in brendancol/xarray-spatial-skills and is synced locally via sync.sh.

• Repoint that link to https://github.com/brendancol/xarray-spatial-skills, matching how AI_POLICY.md and CONTRIBUTING.md already reference it.

Docs-only change, no code paths touched.

Test plan:

• Confirmed .claude/commands/ is gitignored and absent from the committed tree
• Grepped the README for other .claude/.codex/.cursor/.kilo links; this was the only one

[brendancol]

PR Review: Point README AI-review link to xarray-spatial-skills repo

Docs-only change, one line. No code, tests, or backend surfaces touched, so the correctness / backend-parity / performance / test-coverage sections don't apply.

Blockers (must fix before merge)

None.

Suggestions (should fix, not blocking)

None.

Nits (optional improvements)

None.

What looks good

• The old target .claude/commands is gitignored (.gitignore:107), so the link 404d on GitHub. Repointing it fixes a real broken link.
• The new URL https://github.com/brendancol/xarray-spatial-skills matches the canonical references already in AI_POLICY.md:130 and CONTRIBUTING.md:66, so the three docs now agree.
• Link text and target both updated; no dangling .claude/commands mention left in the README (verified by grep).

Checklist

• Broken link resolved to the correct destination
• New URL consistent with AI_POLICY.md / CONTRIBUTING.md
• No other .claude/.codex/.cursor/.kilo links left broken in README
• Algorithm / backend / NaN / dask / benchmark checks — n/a (docs only)