feat: add floci dev container feature

[github-actions]

Summary

• add a Dev Container Feature for the official floci-io/floci-cli release binaries
• support latest-version resolution and pinned versions on Linux amd64/arm64
• add feature, pinned-version, and all-tools test coverage
• register floci in the CI feature matrix and document usage

Validation

• installed and ran the latest release (0.2.3) in a clean ubuntu:latest container
• installed and asserted pinned version 0.2.3 in a clean ubuntu:latest container
• passed Bash syntax checks, JSON parsing, and git diff --check

The local devcontainer features test harness could not be installed because the npm registry repeatedly failed TLS handshakes; the installer paths used by the feature were validated directly in clean containers.