Cut first pinnable release v1.2.0 + publish Agent Skills Discovery Index (RFC v0.2.0)

[kaankacar]

Two coupled changes so this repo becomes a pinnable, agent-discoverable skill provider:

1. Cut the first tagged release v1.2.0 (no tags exist yet; plugin.json is at 1.1.1 but unreleased).
2. Publish an Agent Skills Discovery Index at /.well-known/agent-skills/index.json (RFC v0.2.0) from the site, listing all 7 skills with content digests pinned to v1.2.0.

Downstream, stellar-docs (stellar/stellar-docs#2440) links to this index. Keeping the index here means digests live next to the content they hash.