Merge the fish config fragments back into one config.fish

The interactive init is small; four fragment files was over-splitting it.
Keep the mechanic (a real fish file inlined by Nix into config.fish) but
merge the fragments into a single modules/fish/config.fish read with
builtins.readFile. Rendered config is unchanged.

.claude/tasks/0005-fish-shell-module.md

@@ -39,6 +39,6 @@ Configure the plugins natively through home-manager rather than a fish plugin ma
 - **Abbreviation-first.** Every non-eza alias is now a `shellAbbr` (the eza `ls` family stays an alias), `preferAbbrs = true`, and `generateCompletions = true` is pinned rather than left to the upstream default.
 - **vi command-line editing.** `interactiveShellInit` sets `fish_key_bindings fish_vi_key_bindings`; the `bang-bang` plugin re-binds `!`/`$` in insert mode via its own `--on-variable fish_key_bindings` handler, so the switch keeps them working.
 - **Trimmed aliases.** Navigation capped at four dots (`.....`/`......` dropped); `psmem`, `psmem10`, `dir`, `vdir`, and `please` removed.
-- **Interactive init is split in the repo, assembled by Nix.** The Module lives at `modules/fish/fish.nix`. Its `interactiveShellInit` is concatenated (in order) from concern-scoped fragments under `modules/fish/config/` — `bindings.fish` (vi editing), `env.fish` (`EDITOR`/`VISUAL` and the bat manpager), `done.fish` (the done plugin tuning), and `path.fish` (`~/.local/bin` and `~/.fish_profile`) — via `lib.concatMapStringsSep "\n" builtins.readFile`. Each concern stays an editable fish file, but home-manager still writes one `~/.config/fish/config.fish`; nothing of ours is autoloaded from a separate runtime file. The concatenated result is byte-identical to the previous monolithic `config.fish` (same toplevel hash).
+- **Interactive init lives in a real fish file.** The Module lives at `modules/fish/fish.nix`; its `interactiveShellInit` is `builtins.readFile ./config.fish`, so the interactive init is written as one editable fish file (vi editing, `EDITOR`/`VISUAL`, the bat manpager, the done plugin tuning, and `~/.local/bin`/`~/.fish_profile`) that home-manager renders into `~/.config/fish/config.fish`. The file is small enough that splitting it into fragments was not worth the indirection; Nix inlines it at build time, so nothing of ours is autoloaded from a separate runtime file.
 - **`copy` stays a function file.** `functions/copy.fish` holds the non-trivial `copy` body, read into the `functions` option; fish autoloads function files lazily, so that is the idiomatic home for a function. Trivial one-liner functions stay inline in `fish.nix`.
 - The Auto-loader only collects `.nix`, so every `.fish` file under `modules/fish/` is inert to it.