Clarify Git-based GitHub-primary positioning in README and FAQ

[codeforester]

Goal

Clarify first-reader positioning that Base is Git-based and GitHub-primary.

Background

Issue #1344 and PR #1345 established the canonical source-control and forge support contract. The current detailed docs are still aligned with that contract, including newer workspace commands. The remaining polish gap is at the first-reader layer: the README opening line still leads with "GitHub workspace control plane", and the FAQ does not directly answer whether Base can be used without GitHub.

Scope

• Update the README opening description so it says Base is Git-based and GitHub-primary without implying full forge independence.
• Add a direct FAQ entry answering whether Base can be used without GitHub.
• Link readers to docs/source-control-and-forge-support.md for command-by-command behavior.
• Keep the canonical support contract unchanged.

Acceptance Criteria

• README first-screen copy uses "Git-based" and "GitHub-primary" language.
• FAQ includes a concise "Can I use Base without GitHub?" answer.
• FAQ answer mentions that Base assumes Git, excludes non-Git SCMs, supports local project workflows for already-cloned non-GitHub Git repositories, and keeps GitHub as the only first-class forge automation target today.
• Documentation points readers to the canonical source-control and forge support page.
• No code behavior changes are made.

Validation

• git diff --check
• Focused docs review of README and FAQ links.

Non-Goals

• Add GitLab, Bitbucket, Azure DevOps, or other forge adapters.
• Change repo clone, workspace clone, repo configure, workspace configure, gh, or release-publish behavior.
• Support Mercurial, Perforce, Subversion, Fossil, or other non-Git SCMs.
• Rework the canonical command compatibility table.

Priority

P3

Size

T

Area

Docs

Initiative

Adoption Polish

Agent Assignment

Human first