Add apt generator for available repo packages

[lucieleblanc]

Summary

Add a new list_available_packages generator for apt-based package managers that suggests packages available from configured repositories using apt-cache pkgnames, rather than only installed packages.

Changes

• command-signatures/src/generators/apt.rs: Add list_available_packages generator (uses apt-cache pkgnames) and register it in both apt_get_generators() and aptitude_generators()
• command-signatures/json/apt-get.json: Update install subcommand to use list_available_packages instead of list_all_packages
• command-signatures/json/aptitude.json: Same update for install subcommand

The existing list_all_packages generator (via dpkg-query) is retained for remove/purge subcommands where only installed packages are relevant.

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Conversation: https://staging.warp.dev/conversation/ec0771ee-86a6-413c-8a30-8e6a893cc578
Run: https://oz.staging.warp.dev/runs/019d473b-73ec-7aa9-8fb2-cd5efefccc49
This PR was generated with Oz.

[linear]

APP-3505 Add apt generator for available repo packages

Spec + generator exist (list_all_packages via dpkg-query). Current generator only lists installed packages.

Remaining work: Add generator for available repo packages (e.g. via apt-cache pkgnames).

[lucieleblanc]

Generator Validation Screenshots

Tested the list_available_packages generator (backed by apt-cache pkgnames) on a local Warp build with this branch's changes patched in.

apt-get install completions

The completions dropdown shows available packages from the system's package cache, each labeled as "package":

aptitude install completions

Same generator works for aptitude install:

Both generators are working correctly — the completions dropdown appears with available package names sorted alphabetically when pressing Tab after install .