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>

README.md

@@ -102,6 +102,46 @@ which Gadfly also honors — but those are **HTTPS-only**, so for plaintext loca
 > **Gitea Actions note:** repo `vars`/`secrets` aren't auto-exposed as env — add each alias to
 > the stub workflow's `env:` block, e.g. `GADFLY_ENDPOINT_BIGBOX: ${{ vars.GADFLY_ENDPOINT_BIGBOX }}`.
 
+## Specialists (the review swarm)
+
+Instead of one generic reviewer, Gadfly runs a **suite of specialists** — each a focused lens
+with its own review (+recheck) pass — and merges them into **one comment**, a collapsible
+section per lens, led by an overall verdict (the worst across lenses; the optional
+`improvements` lens never escalates it).
+
+**Default suite** (when nothing is configured):
+`security`, `correctness`, `maintainability` (code cleanliness), `performance`, `error-handling`.
+
+**Also built in** (opt-in by name): `tests`, `docs`, `conventions`, and `improvements`
+(strict & quiet — at most 1–2 high-value, non-blocking suggestions, silent otherwise).
+
+Select which run with **`GADFLY_SPECIALISTS`** (comma-separated names, or `all`):
+
+```yaml
+GADFLY_SPECIALISTS: "security,correctness,maintainability,tests"
+```
+
+**Define your own** — two ways, which compose (env overrides file overrides built-ins):
+
+```yaml
+# 1. env: GADFLY_SPECIALIST_<NAME>="<focus>"  (also overrides a built-in by reusing its name)
+GADFLY_SPECIALIST_MIGRATIONS: "Review DB migrations for destructive or unindexed changes."
+GADFLY_SPECIALISTS: "security,correctness,migrations"
+```
+
+```yaml
+# 2. a repo .gadfly.yml at the repo root (version-controlled). See examples/.gadfly.yml:
+specialists: [security, correctness, maintainability, migrations]
+define:
+  - name: migrations
+    title: "🗃️ DB migrations"
+    focus: "Review schema migrations for destructive ops, missing indexes, table locks."
+```
+
+> **Cost:** each specialist is its own review+recheck, so cost ≈ *specialists × models × 2*.
+> The default suite runs on a **single** model. Trim with `GADFLY_SPECIALISTS`, and a future
+> `auto` mode will let a cheap model pick only the lenses a given diff actually needs.
+
 ### Triggers
 
 1. A **new/reopened/ready** non-draft PR — automatic.