install: add fish shell support for PATH configuration

[marcelsafin]

Fish shell users currently fall into the catch-all *) case in the shell profile detection, which writes POSIX export syntax to ~/.profile. This silently does nothing because:

1. Fish does not source ~/.profile
2. Fish does not use export PATH="...:$PATH" syntax (PATH is an array in fish, not colon-separated)
3. The get-started hint export PATH="..." && copilot help fails in a fish session

This adds a fish) case alongside the existing zsh) and bash) cases, using the idiomatic config path and command:

• Config file: ${XDG_CONFIG_HOME:-$HOME/.config}/fish/conf.d/copilot.fish (auto-sourced by fish)
• PATH command: fish_add_path (available since fish 3.2, released March 2021)

The three places that previously hardcoded export PATH=... now use a PATH_LINE variable, so the shell-specific logic lives in one place rather than being duplicated across the interactive prompt, non-interactive hint, and get-started instructions.

Before (fish user):

Would you like to add it to /home/user/.profile? [y/N] y
✓ Added PATH export to /home/user/.profile    ← fish never reads this
  export PATH="/home/user/.local/bin:$PATH" && copilot help    ← fails in fish

After (fish user):

Would you like to add it to /home/user/.config/fish/conf.d/copilot.fish? [y/N] y
✓ Added PATH export to /home/user/.config/fish/conf.d/copilot.fish
  fish_add_path "/home/user/.local/bin" && copilot help

No change in behavior for bash, zsh, or other POSIX shells.

[Copilot]

Pull request overview

Adds first-class fish shell handling to the installer’s PATH configuration logic so fish users get a fish-compatible PATH command and the correct config file path.

Changes:

• Detects fish as a distinct shell and targets ~/.config/fish/conf.d/copilot.fish (or $XDG_CONFIG_HOME equivalent).
• Centralizes the shell-specific PATH command into a PATH_LINE variable used for interactive edits and printed hints.
• Updates the installer’s non-interactive and “get started” messages to use PATH_LINE instead of hardcoded export PATH=....

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[Copilot]

For the fish case, RC_FILE points into ${XDG_CONFIG_HOME:-$HOME/.config}/fish/conf.d/, which may not exist. Appending with >> "$RC_FILE" will fail with “No such file or directory”. Consider creating the parent directory (e.g., mkdir -p "$(dirname "$RC_FILE")") before writing or before printing instructions that reference this path.

[Copilot]

Pull request overview

Copilot reviewed 1 out of 1 changed files in this pull request and generated no new comments.

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[devm33]

Thanks again!