Route devel plugin AI tooling through pkg/use, not raw :straight wiring

[yottanami]

Summary

plugins/devel/core.el had leftover pre-Nix package wiring that never got migrated when straight.el was dropped (9822f0a):

• copilot went through pkg/use (which strips :straight and forces :ensure nil) but also carried an explicit :ensure t in its details. Since pkg/use expands to (use-package copilot :ensure nil ...details...), that explicit :ensure t comes after the macro's own :ensure nil, and use-package's plist normalization takes the last occurrence of a keyword — so :ensure t wins, silently defeating the wrapper.
• shell-maker and copilot-chat used bare use-package with :straight recipes referencing sources straight.el no longer fetches from, instead of pkg/use like every other package in the file.

What I could confirm, and what I couldn't (no nix/emacs binary in my environment):

• Confirmed by reading: copilot is absent from nix/emacs.nix's package list; shell-maker/copilot-chat are present. copilot.el is on MELPA, and nixpkgs auto-generates emacsPackages recipes for MELPA, so it's addable the same way copilot-chat already is (not a from-source build like helm-ag).
• Not confirmed by a live run (issue explicitly asks for this): whether copilot was actually broken before this fix. My reasoning it likely was: with :ensure t winning and the :straight recipe being its only real fetch source, use-package would fall through to package.el's default install path, which needs refreshed package-archive-contents — and this build's pkg/initialize deliberately never sets that up ("no network access, no package manager bootstrap", per its own doc comment). That's static reasoning about documented use-package/package.el behavior, not an observed failure — please verify with a real build.

Changes

• copilot: dropped the vestigial :straight recipe and the harmful redundant :ensure t, letting pkg/use's :ensure nil actually take effect.
• shell-maker / copilot-chat: converted to pkg/use, dropped their :straight recipes.
• nix/emacs.nix: added copilot to the package list.

Out of scope, flagged for a possible follow-up

plugins/editor/core.el:52 (smart-mode-line) has the same kind of leftover :straight recipe, but it's a different file and wasn't named in this issue's acceptance criteria, so I left it alone — harmless today since pkg/use strips it regardless, just the same style inconsistency.

Closes Men-in-Black-5/ideas#19

Test plan

• Parenthesis-balance check on the edited file — balanced
• grep -rn ":straight" — no more :straight recipes in plugins/devel/core.el
• nix flake check / a live build confirming copilot, copilot-chat, shell-maker all load — not possible in my environment, needs a real check

[yottanami]

Reviewed and went a bit further than static reading since I can't run Emacs/Nix either (same environment gap as the PR author): confirmed the core technical claim two ways.

1. The :ensure t-wins claim: fetched use-package's actual source (use-package-core.el, use-package-normalize-plist) — when a keyword is specified twice, it's re-set via plist-put, so the later occurrence overwrites the earlier one. Since pkg/use splices details in after its own :ensure nil, an explicit trailing :ensure t in details really does win, exactly as described. Not just plausible — confirmed against the library's own merge logic.
2. emacsPackages.copilot existing: found pkgs/applications/editors/emacs/elisp-packages/manual-packages/copilot/package.nix in nixpkgs via GitHub code search — it's a real, manually-packaged (not MELPA-auto-generated) entry, so adding it to nix/emacs.nix's package list the same way copilot-chat already sits there is valid.

Also verified: balanced parens on the edited file, no remaining :straight references in plugins/devel/core.el, and the shell-maker/copilot-chat conversions to pkg/use are mechanical and match the file's existing style.

Same situation as PR #1 though: no agent can actually load this in Emacs or run nix flake check, so per the current policy this waits for @yottanami to confirm a real build/load before I merge — everything I can verify checks out.