feat: shell completions — bash, zsh and fish (v1.7.0)

[shouze]

Context

Part of the 🛠 Developer experience EPIC #77, targeting v1.7.0.

Goal

Add a completions subcommand that outputs a shell completion script for bash, zsh or fish.

Users install once:

# bash (~/.bashrc)
eval "$(github-code-search completions --shell bash)"

# zsh (~/.zshrc)
eval "$(github-code-search completions --shell zsh)"

# fish (~/.config/fish/config.fish)
github-code-search completions --shell fish | source

When --shell is omitted, the shell is auto-detected from $SHELL.

Scope

• New completions subcommand in github-code-search.ts
• Completion scripts as inline templates (no new runtime dependency)
• Covers subcommands: query, upgrade, completions
• Covers common options: --org, --format, --output-type, --exclude-repositories, --exclude-extracts, --group-by-team-prefix, --no-interactive, --include-archived, --no-cache
• TDD: unit tests that verify the output is a non-empty string containing the tool name for each shell

Acceptance criteria

• github-code-search completions --shell bash → valid bash script
• github-code-search completions --shell zsh → valid zsh script
• github-code-search completions --shell fish → valid fish script
• Auto-detection from $SHELL (fish/zsh/bash)
• Unknown shell → friendly error message + exit 1
• Unit tests pass