docs(contributing): scope contribution areas to the CLI

[cristim]

Closes #1177

Summary

CONTRIBUTING.md listed some contribution areas that this repo doesn't own. This PR changes the "Areas for Contribution" section to point each of them at the repo that does:

• The web dashboard, REST API and deployment work (Terraform, CloudFormation) live in cloud-commitments-platform. I removed them from the lists and added one pointer paragraph.
• The AWS, Azure and GCP provider and service implementations live in cloud-commitments-go, as go.mod and CONTRIBUTING.md:204 already say. That High Priority item now reads "Command and flag support for additional services", with a link to that repo.

The issue's other findings were already fixed on main:

• CONTRIBUTING.md:51 and :88 say Go 1.26.9, matching go.mod.
• README.md:81-83 scopes the README to the CLI and links the platform repo.
• Neither file says go install .../cmd@latest any more.

Evidence

• An independent non-author reviewer (sonnet, high) checked every claim in the issue against origin/main, then reviewed the diff in two passes. One low finding, about the provider items, was applied verbatim. The final check had no actionable findings at cfabc29.
• markdownlint and the other pre-commit hooks pass.

🤖 Generated with Claude Code

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Reviewing files that changed from the base of the PR and between e8543f0 and cfabc29.

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• CONTRIBUTING.md

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[cristim]

Independent review verdict: CLEAN at cfabc29

Reviewer: fresh Claude agent (Sonnet, high effort), not the author. Coordinator: claude-cc-reviewer.

What was checked:

• The new links and the cmd/, docs/, scripts/, go.mod and Makefile paths exist in the repo.
• The text agrees with README.md:79-83 and CONTRIBUTING.md:201/207/218.
• No stale web UI, REST API, Terraform or CloudFormation areas remain. The one remaining platform mention (CONTRIBUTING.md:287) is a legitimate cross-repo reference.
• markdownlint v0.47.0 (the pinned version) passes.

Gate:

• CI is green, and mergeability is CLEAN.
• Labels are mirrored from HYG-08: README/CONTRIBUTING describe a CLI-only project on Go 1.23; web platform and Go 1.25 absent or contradicted #1177.
• CR waived: quota, adversarial review + local verification + green CI. (claude-cc-reviewer)