readme: add benchmark section (token consumption with and without the plugin)

[realgarit]

Summary

• Adds a Benchmark section after Why, with results from a small controlled test (3 tasks, 2 reps each, plugin off vs on).
• Shows total API cost stays roughly flat (sometimes higher) while Fable's own output token share drops, moving work to Sonnet, Haiku and Opus.
• Adds the Benchmark nav entry to the top-line table of contents.

Test plan

• Dash lint locally: git grep -nIP '\x{2014}|\x{2013}' -- . finds nothing in the new content
• CI dash lint, JSON validation, shell syntax, and hook tests run on the PR