feat: specialist suite — configurable + custom review lenses (one consolidated comment)

Replace the single generic review with a suite of focused specialists, each its
own review+recheck pass, merged into ONE comment (a collapsible section per lens,
led by the worst verdict; the optional `improvements` lens never escalates it).

- cmd/gadfly/specialists.go: built-in lenses + default suite (security, correctness,
  maintainability, performance, error-handling) + opt-in (tests, docs, conventions,
  improvements). Selection via GADFLY_SPECIALISTS (csv/"all"); custom defs via
  GADFLY_SPECIALIST_<NAME> env and a repo .gadfly.yml (specialists + define).
  Precedence: built-ins < file < env. Unknown names error but don't sink the run.
- cmd/gadfly/consolidate.go: verdict parse + one-comment render.
- main.go: loop specialists; per-lens failure is an inline notice, never fatal.
  Default timeout bumped to 600s (suite runs sequentially).
- base system prompt trimmed to persona+tools+discipline+output; lens-specific
  focus is appended per specialist (semantic re-derivation discipline kept in base).
- entrypoint default models -> single model (suite already gives breadth; cost ~=
  specialists × models × 2). Adds gopkg.in/yaml.v3.
- docs/examples: README "Specialists" section, examples/.gadfly.yml, stub var,
  CLAUDE.md architecture/config. Dynamic `auto` selection is the planned next step.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

go.mod

@@ -2,7 +2,10 @@ module gitea.stevedudenhoeffer.com/steve/gadfly
 
 go 1.26.2
 
-require gitea.stevedudenhoeffer.com/steve/majordomo v0.0.0-20260610113006-0147a79d187b
+require (
+	gitea.stevedudenhoeffer.com/steve/majordomo v0.0.0-20260610113006-0147a79d187b
+	gopkg.in/yaml.v3 v3.0.1
+)
 
 require (
 	cloud.google.com/go v0.116.0 // indirect