feat: re-platform agentic review onto executus + large-PR cost controls (#20)

Makes gadfly a consumer of executus (run.Executor compaction/bounding/budget/critic + fanout) and fixes the large-PR token burn in size-gated layers: paginated get_diff, downshift above GADFLY_HUGE_DIFF_BYTES, and a swarm-wide GADFLY_PR_BUDGET_SECS backstop. Small PRs untouched; advisory-only and the static binary preserved. Dogfood swarm reviewed it (6 models, 21 real findings graded + folded in).

Co-authored-by: Steve Dudenhoeffer <steve@stevedudenhoeffer.com>
Co-committed-by: Steve Dudenhoeffer <steve@stevedudenhoeffer.com>

Dockerfile

@@ -24,7 +24,10 @@ RUN --mount=type=cache,target=/go/pkg/mod \
     go build -trimpath -ldflags="-s -w" -o /out/gadfly ./cmd/gadfly
 
 FROM alpine:3.20
-RUN apk add --no-cache bash git curl jq ca-certificates nodejs npm
+# procps provides pkill/pgrep, which entrypoint.sh's per-PR wall-clock backstop
+# (GADFLY_PR_BUDGET_SECS) uses to stop the review subtrees — busybox's applets
+# are not guaranteed to include them.
+RUN apk add --no-cache bash git curl jq ca-certificates nodejs npm procps
 # Bundle the Claude Code CLI so the `claude-code` review engine works out of the
 # box (GADFLY_MODELS=claude-code or claude-code/<model>). This adds Node + the
 # CLI to the image (notably larger); ollama-only users pay the size but nothing