feat: re-platform agentic review onto executus + large-PR cost controls

Fixes the large-PR token burn: a ~250K-token diff was re-sent every agent
step across models × lenses × passes, draining a metered usage block in
minutes. Small PRs are untouched (every mitigation is size-gated / no-op
under threshold).

- Re-platform the in-process review path onto executus run.Executor: context
  compaction (executus/compact, threshold from the model's real context window
  via executus/model), run-bounding, a per-PR budget gate (Ports.Budget), and
  the wrap-up nudge re-expressed as a run.Critic. Lens fan-out now uses
  executus/fanout. gadfly keeps its own model.go, so GADFLY_ENDPOINT_<NAME>
  aliases and the claude-code engine are unaffected. No majordomo bump; the
  binary stays static (executus core is majordomo+stdlib only).
- Paginate get_diff (per-file `path` + start_line/limit) instead of dumping the
  whole diff; trim the recheck diff embed (60k -> 20k chars).
- entrypoint.sh: downshift the fleet above GADFLY_HUGE_DIFF_BYTES (one cheap
  model, fewer lenses/steps, no recheck) + a swarm-wide GADFLY_PR_BUDGET_SECS
  wall-clock backstop (adds procps for pkill). All advisory; CI never fails.
- README + CLAUDE.md + tests updated.

Note: run.Result exposes no transcript, so the old transcript-based forced-
finalization fallback is dropped; the wrap-up critic nudge is the remaining
"always emit something" mechanism.

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

go.mod

@@ -3,6 +3,7 @@ module gitea.stevedudenhoeffer.com/steve/gadfly
 go 1.26.2
 
 require (
+	gitea.stevedudenhoeffer.com/steve/executus v0.1.4
 	gitea.stevedudenhoeffer.com/steve/majordomo v0.0.0-20260627225659-aa25b2c33462
 	gopkg.in/yaml.v3 v3.0.1
 )
@@ -17,6 +18,7 @@ require (
 	github.com/go-logr/stdr v1.2.2 // indirect
 	github.com/google/go-cmp v0.7.0 // indirect
 	github.com/google/s2a-go v0.1.9 // indirect
+	github.com/google/uuid v1.6.0 // indirect
 	github.com/googleapis/enterprise-certificate-proxy v0.3.17 // indirect
 	github.com/googleapis/gax-go/v2 v2.22.0 // indirect
 	github.com/gorilla/websocket v1.5.3 // indirect