docs: document Fish completion installation - #40

Summary

• Document the already-shipped Fish completion command in the README CLI
  reference and deployment summary.
• Add a copy-paste Fish completion installation path under Fish's normal user
  completion directory.

No completion generator, shell profile, service authority, or runtime behavior
changes.

Validation

• npm run typecheck passed.
• npm run build passed.
• npm run check:package and npm run check:site passed.
• npm audit --omit=dev passed with no vulnerabilities.
• npm pack --dry-run --ignore-scripts passed with 162 files.
• The local Node 25 suite reported 64 passed, 1 skipped, and one existing ARM
  browser-discovery failure; the docs-only change does not touch that surface.
• git diff --check passed after restoring the docs file that the site check
  regenerates.

Closes #10