feat: replace ag with ripgrep and retain ag command

[mikaello]

Change

Replace The Silver Searcher with maintained ripgrep and bump the feature to v3.
Install an ag command pointing to rg when ag is absent, preserving basic searches such as ag PATTERN [PATH].
Some ag options and output differ from rg; for example, the two tools use -g differently.
Use distribution packages for all four tools.
Support Debian 13+ and Ubuntu 24.04+ without the old external APT repository fallback.
Keep numbered installation progress messages.
The old USERNAME lookup did not affect any installation path or ownership.
The feature-starter installer confirms that feature installation runs as root and supplies final-user variables when needed.

Validation

• bash -n passes for installer and test script.
• Feature metadata parses as JSON, the workflow parses as YAML, and git diff --check passes.
• CI tests Debian 13, Ubuntu 24.04, and the Dev Containers Ubuntu base image.
• Tests check rg, the ag compatibility command, and a basic ag search.

Sources

• https://github.com/devcontainers/feature-starter
• https://containers.dev/guide/feature-authoring-best-practices/
• https://packages.debian.org/trixie/eza
• https://packages.ubuntu.com/noble/eza
• https://github.com/BurntSushi/ripgrep/blob/master/FAQ.md