feat(shell): add fish shell plugin

[tivris]

Full port of the zsh shell plugin to fish, giving fish users the same :command experience.

What's included

Fish scripts (fish-plugin/, 57 files):

• Dispatcher bound to Enter key — parses :command [args] and routes to action handlers
• Tab completion for :commands (via fzf) and @[file] references (via fd/bat)
• Right prompt showing agent, model, token count, and cost
• All action handlers: conversation management, agent/model/provider selection, git integration, editor, workspace sync, etc.
• Doctor diagnostics and keyboard shortcut reference adapted for fish
• Setup template for config.fish injection

Rust integration (crates/forge_main/src/fish/, 4 files):

• FishCommandGroup with plugin, theme, doctor, rprompt, setup, keyboard subcommands
• FishRPrompt using ANSI escapes instead of zsh's %F{} prompt escapes
• Plugin generation via include_dir! embedding + clap_complete::shells::Fish
• Setup targeting ~/.config/fish/config.fish with XDG_CONFIG_HOME support

Usage

# one-time setup
forge fish setup

# or manually
forge fish plugin | source
forge fish theme | source

# then just use it
: hello world
:conversation
:new
:commit

Testing

• All 57 .fish files pass fish -n syntax validation (tested on fish 4.6.0)
• 27 Rust tests covering rprompt output, style escapes, CLI parsing, setup/marker logic, plugin/theme generation
• Functional test: plugin loads in fish, all 54 _forge_* functions register, logging/doctor work end-to-end

Closes #2765

[tusharmath]

@tivris have you tried this PR? Does it work? Too big of a PR to review 🙈.

[tivris]

@tivris have you tried this PR? Does it work? Too big of a PR to review 🙈.

Tested it locally with fish 4.6.0 on macOS (Ghostty). Setup, plugin loading, theme, completions, key bindings, and : commands all work, ran forge fish setup, reloaded the shell, and confirmed with : Hi. It's a bit fishy

[tusharmath]

Related to #2839

A thought:

• Expose fish unofficially via a user plugin
• Setup discord specifically for ZSH, FISH and Powershell for people to collaborate and provide feedback until is stable enough to become a member of the forgecode ecosystem officially.

This way we don't need to bloat our repos/releases and we can support them independently.

[guptakanishka90]

Desperately waiting for this feature.

[iosif2]

Related to #2839

A thought:

• Expose fish unofficially via a user plugin
• Setup discord specifically for ZSH, FISH and Powershell for people to collaborate and provide feedback until is stable enough to become a member of the forgecode ecosystem officially.

This way we don't need to bloat our repos/releases and we can support them independently.

I’ve been experimenting with a possible unofficial fish shell integration on my side.

It tries to mirror the general approach of the existing zsh integration, but it’s still very early and likely incomplete.

Sharing in case it helps inform how external shell integrations might be structured:
https://github.com/iosif2/forge.fish

[github-actions]

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