feat(core): use public api url in managed prompts

[skulidropek]

Summary

• add DOCKER_GIT_API_PUBLIC_URL to managed AGENTS.md, CLAUDE.md, and GEMINI.md prompts
• tell auto-launched agents to use the public docker-git API URL and never invent localhost / 127.0.0.1 links
• persist the public API URL into SSH environment setup for container sessions
• pass DOCKER_GIT_API_PUBLIC_URL through docker-compose.api.yml
• document the public API URL env in the API README
• add coverage for prompt rendering and compose wiring in app/lib tests

Verification

• pnpm install --frozen-lockfile
• pnpm --filter ./packages/lib exec vitest run tests/core/templates.test.ts
• pnpm --filter ./packages/app test -- tests/docker-git/entrypoint-auth.test.ts
• pnpm --filter ./packages/lib typecheck && pnpm --filter ./packages/app typecheck

Proof Obligations

• Invariant: if DOCKER_GIT_API_PUBLIC_URL is set, managed prompts surface that exact public URL
• Invariant: managed prompts never instruct agents to use localhost / 127.0.0.1 as the public docker-git API address
• Invariant: auto-agent startup preserves the same public-URL guidance as the managed prompt files
• Invariant: the API compose entrypoint and docs expose the same public-URL configuration path

Closes #78