claude-code+codex+opencode+gemini-cli: unify context and skills options

[iniw]

Description

these assistant modules exposed the same concepts through different option names and types. global instructions lived under memory, custom-instructions, or rules, and claude code split skill configuration between skills and skillsDir. that made shared configurations harder to move between assistants and left claude code less flexible than the others.

this patch adds a canonical context option for the shared AGENTS-style file and keep the old option names as compatibility shims with migration warnings.
also let programs.claude-code.skills accept either an attribute set or a directory path so it matches the bulk-directory workflow already supported elsewhere.

it also updates the module tests and add a news entry so the migration path is explicit.

Checklist

• Change is backwards compatible.

• Code formatted with nix fmt or
  nix-shell -p treefmt nixfmt deadnix keep-sorted nixf-diagnose --run treefmt.

• Code tested through nix run .#tests -- test-all or
  nix-shell --pure tests -A run.all.

• Test cases updated/added. See example.

• Commit messages are formatted like

  {component}: {description}
  
  {long description}
  

  See CONTRIBUTING for more information and recent commit messages for examples.

• If this PR adds a new module

  • Added myself as module maintainer. See example.
  • Generate a news entry. See News
  • Basic tests added. See Tests

• If this PR adds an exciting new feature or contains a breaking change.

  • Generate a news entry. See News

[khaneliman]

I had an alternative method in a local branch that I haven't gotten back to. Might take a look again soon

[iniw]

I had an alternative method in a local branch that I haven't gotten back to. Might take a look again soon

Should I close this PR then?

[iniw]

Added support for gemini-cli.skills to be set to a directory, an addition to #9009 now that it's merged

[khaneliman]

I had an alternative method in a local branch that I haven't gotten back to. Might take a look again soon

Should I close this PR then?

Sorry, been super busy and forgot to respond. I actually think this is a fine first step for what I was planning around these modules that all share similar concepts. I don't think it conflicts with the general concept so we can get this in first to improve things from a UX perspective.