fix: update terraform docs for provider-docs and provider-test-patterns skills

[AdamTylerLynch]

The repo listings drifted out of date as skills were added:

• terraform/README.md — provider-docs was missing from the
  terraform-provider-development table, the npx skills add list, and the structure
  tree
• AGENTS.md — both provider-docs and provider-test-patterns were missing from
  the structure tree, the plugin-contents table, and the npx skills add list
• CHANGELOG.md — neither skill had an entry; both added under Unreleased/Added
  (the 0.1.0 "9 total skills" line was accurate at the time and is left alone)

Docs-only change; no skill content touched. Deliberately does not mention the new
provider-configuration skill — its own PR self-registers, keeping both PRs correct
in any merge order (expect a trivial adjacent-line conflict in these three files with
that PR; both-sides-keep resolves it).

Verification: scripts/validate-structure.sh passes.

[leefowlercu]

Thank you for this contribution, @AdamTylerLynch. The provider documentation and test Skill registrations were incorporated into #90 through commit 2031803, and #90 has now merged. Closing this pull request as superseded by #90.