docs: document the shipped Fish completion command

[AjnasNB]

Current state

The completion generator, CLI parser, and snapshot coverage for Bash, Zsh, Fish, and PowerShell already ship on main:

• src/operator-install.ts implements fishCompletion();
• src/cli.ts accepts completion fish;
• test/operator-install.test.ts exercises all four generators.

The remaining gap is public onboarding: README.md lists only Bash, Zsh, and PowerShell in its quick examples, CLI reference, and deployment-capability summary.

Contributor-sized task

Update only the public completion documentation so Fish users can discover and install the already-shipped command.

Acceptance criteria

• Add cockroach-browser completion fish to the completion quickstart in README.md.
• Update the CLI reference signature to <bash|zsh|fish|powershell>.
• Include Fish in the README deployment-capability summary.
• Add a copy-paste Fish installation example to docs/operator-install.md using Fish's normal user completion directory.
• Do not change completion generation, shell profiles, service installation, runtime authority, or security policy.
• Run npm run check and include the result in the pull request.

Start here

Read CONTRIBUTING.md, then edit README.md and docs/operator-install.md. This issue is intentionally documentation-only and does not require Chromium.