Explain why BitterGit exists

[ruemic]

Summary

• explain the original product motivation: a GitHub account should not be required before an agent can start useful work
• connect the open-source server to Bitter and BitterGit.com without conflating hosted services with this repository
• align package metadata and discovery links with the public project

Verification

• bun install --frozen-lockfile
• bun test
• bun run typecheck
• bun run build
• git diff --check
• public link checks