docs: fix agent skills index link

[mikemikimike]

Summary

Fixes #479.

The Skills README linked to https://docs.stripe.com/.well-known/skills, which is a dead endpoint. Update it to Stripe's canonical machine-readable Skills index: https://docs.stripe.com/.well-known/skills/index.json.

Reproduction and resolution

• Before this change, the Issue URL returned HTTP 404.
• After this change, the replacement URL returns HTTP 200 and serves the Skills index.
• The replacement is an index JSON endpoint by design; it is the discovery endpoint consumed by Skills tooling, rather than an HTML documentation page.

Compatibility and risk

Documentation-only change. No runtime code, API behavior, dependencies, sync logic, or generated files are changed. There are no state, concurrency, atomicity, or migration concerns.

Testing

• git diff --check — passed.
• Original endpoint — reproduced HTTP 404.
• Replacement endpoint — verified HTTP 200 with curl.

Build and unit tests were not run because this PR changes only one Markdown URL and has no executable code path. Automated link-checking could be added separately, but is outside the scope of this Issue.

[cla-assistant]

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[mikemikimike]

recheck