docs: refresh the built-in specialist README

[ZeroPoint95]

What changed

• Document /senior-dev, /sec and /review with consistent purposes, example commands and outputs.
• Replace the obsolete specialist illustration and coming-soon review claim.
• Credit PR-AF and SEC-AF, link the original review benchmark, and distinguish its results from codeaf measurements.
• Preserve supported custom saved harnesses. Leave the tentative swe-pro-go attribution out pending review.

How it was checked

• Existing naming-law tests and git diff --check pass.
• GOMAXPROCS=2 GOFLAGS=-p=2 make pr-ready passes: build, vet, formatting, packed/manual checks, laws, carried-help and belt-page checks; there are no changed Go packages.
• The first gate attempt hit a test fixture cleanup race (directory not empty). The named test passed three fresh runs, and the complete gate passed on rerun.

Checklist

• Change entry included: docs/changes/unreleased/1840-specialist-readme.md; make changelog-check passes.
• No product commands or behavior changed; existing manuals were used as the source.
• No test exemptions added.
• Only explicit documentation paths staged.