Document forge support boundaries

[codeforester]

Summary

• Add a canonical source-control and forge support contract for Base.
• Clarify that Base is Git-based and GitHub-primary, while the local project loop works for non-GitHub Git repositories.
• Fix workspace manifest docs so generic Git URLs are documented as reporting metadata while automatic materialization remains GitHub-only today.

Validation

• env -u BASE_HOME PYTHONPATH=lib/python:cli/python "$HOME/.base.d/base/.venv/bin/python" -m pytest tests/test_base_cli_docs.py tests/test_contract_hardening.py -q — 10 passed
• git diff --check origin/main...HEAD — passed
• env -u BASE_HOME BASE_BASH_LIBS_DIR=/Users/rameshhp/work/base-bash-libs/lib/bash BASE_CACHE_DIR=/private/tmp/base-1344-final-rebased ./bin/base-test — 734 passed, 1 skipped; Bats/source suite 583 passed

Closes #1344