Initial migration of BuilderLab CLI from Berd

[jsibbison-square]

Summary

• migrate the CLI from the Berd repository into standalone builderlab-cli
• rename the product and user-facing command surface to BuilderLab / bl
• add repository documentation, contribution workflow, issue templates, CODEOWNERS, and security policy
• preserve the backend X-BB-Session-Credential wire contract during the product rename

Validation

• just ci
• 166 unit tests
• 81 BuilderLab E2E tests
• 22 CLI E2E tests
• 6 Docker acceptance contract tests
• release build and smoke check

[nathan-thillairajah]

🤖 Pushed f7a9ac2 with focused compatibility fixes for the rename: preserve the BBIdentity authorization scheme, backend Playpen key and builderbot extension ID, plus existing keychain IDs and skill/agent ownership metadata. The command remains bl. The README now explains how to reuse existing bb state and the inherited backend default.

Hermit-managed just ci passes on this head (275 tests, formatting/Clippy, release build and package smoke). The stacked #3 includes this commit and adds auth-crate tests, bl smoke checks and Docker acceptance in CI. No merge performed.