docs(cli): explain unreleased Git activation and recovery

[leoafarias]

Description

Document Git activation for the unpublished CLI and recovery from --source path activation failures. A live checkout activation can change the monorepo lockfile; restoring it can make the command fail before Remix starts. Git activation resolves the CLI independently of that checkout.

Update the CLI README and open-code references. No runtime code changes.

Related Issues

None linked.

Validation

• Reproduced exit 65 after restoring the workspace lockfile on current main (dfba501).
• Git activation at that revision, followed by init and add button, passed in fresh Vanilla and Fortal apps using published Remix 1.0.0-beta.10.
• dart run melos run docs:check — passed.
• git diff --check — passed.

Checklist

• New unit/integration tests (documentation-only; real installation commands verified above).
• Relevant documentation updated.
• Review follow-up pending.

Breaking Change

• Yes, this is a breaking change.
• No, this is not a breaking change.