host P0.2: publish-DAG + execute-fold runner + capability check (hypothesis confirmed)

The hypothesis test. FINDING: a synchronous business flow expresses NATURALLY as an EXECUTE-FOLD
composition (host/execute.sx: seq/effect/alt — the category branch IS 'alt'), NOT an artdag
DATAFLOW DAG (which has no control flow). So 'business logic = art-dag' holds at the ABSTRACTION
(both content-addressed op-DAGs) and is REFINED at the vocabulary: the synchronous control-flow
runner is the execute-fold (caps {effect,branch,each}); artdag is the dataflow sibling. Two
instances of one thing, run very differently — exactly the framing.

lib/host/flows.sx: capability typing (host/flow--node-cap/required-caps derive a DAG's capability
set from its node vocabulary; effect→effect, alt→branch, each→each, wait→suspend), the execute-fold
seam runner (advertises {effect,branch,each}), and host/flow--bind (required ⊆ advertised → derive
the runner, else fail-fast). host/blog--publish-dag (the publish workflow) + publish-ctx.

Verified: publish-DAG required-caps = {effect,branch} → binds to the sync runner; runs →
newsletter→[validate,digest] / urgent→[validate,notify] / other→[validate,skip]; a  node →
{suspend} → binds FAIL-FAST against the exec-runner (would need the Erlang runner, RA). Runner is
DERIVED, not chosen. flows 7/7, blog 203/203, full host conformance 591/591.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

lib/host/blog.sx

@@ -128,6 +128,22 @@
          :id (host/blog-cid slug)                     ;; the object's content CID
          :object {:type "article" :slug slug
                   :category (host/blog--post-category slug)}}))))
+;; P0.2: the publish WORKFLOW as an EXECUTE-FOLD composition (host/execute.sx) — the SYNCHRONOUS
+;; business flow. Validate, then BRANCH on category (newsletter → build a digest, urgent → notify
+;; now, else skip). Content flow (effect/alt), NOT dataflow — so it's the execute-fold, not artdag.
+;; Its required capabilities are {effect, branch} (host/flow--required-caps) → binds to the sync
+;; execute-fold runner (which advertises {effect, branch, each}). A `wait` node would add {suspend}
+;; and fail-fast against that runner (requiring the Erlang runner, RA). Runs against a ctx built
+;; from the activity's object.
+(define host/blog--publish-dag
+  (quote (seq
+    (effect validate (field "slug"))
+    (alt (when (eq "category" "newsletter") (effect digest (field "slug")))
+         (when (eq "category" "urgent")     (effect notify (field "slug")))
+         (else (effect skip))))))
+;; the ctx a publish activity presents to the publish-DAG (string keys — preds read ctx by key).
+(define host/blog--publish-ctx
+  (fn (activity) (let ((o (get activity :object))) {"category" (get o :category) "slug" (get o :slug)})))
 
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