feat(run): Ports.FinalGuard — host seam that nudges an announce-then-stop run to deliver its artifact

A run can render an artifact into host storage, end on a text-only stop
turn claiming "Done — sent you the file", and never call the host's
delivery tool — the artifact strands while the user is told it shipped
(mort general run e9e7bf40 2026-07-14, gifsmith run 29170c93 2026-07-12).
The loop has no seam for the host to intervene at the stop turn.

Add Ports.FinalGuard: consulted once when a single-loop run is about to
finalize successfully. A non-empty nudge is appended to the SAME
conversation as the next user turn and the agent runs ONE bounded extra
round (same toolbox/observers, capped at finalNudgeMaxSteps=6) — enough
to deliver the stranded file or correct the final text, not enough to
start a new work spree. Guard panics and nudge-round failures are
isolated: the original successful result always survives. The nudge is
recorded as a final_nudge audit event and appended to the durable
checkpoint transcript.

stageInputFiles now also returns the staged file ids, surfaced to the
guard via FinalState.StagedFileIDs — staged inputs live under run scope
but are the user's attachments, not run-produced artifacts, so an
undelivered-artifact check must exclude them. FinalState.ToolNames lets
a host skip runs that have no delivery tool at all.

Co-Authored-By: Claude Fable 5 <[email protected]>

run/ports.go

@@ -54,6 +54,11 @@ type Ports struct {
 	// loader tool. nil = SkillPacks are inert. The executus/skillpack battery
 	// ships a default impl (skillpack.Activator).
 	SkillPacks SkillPackActivator
+	// FinalGuard is consulted when a single-loop run is about to finalize
+	// successfully; a non-empty nudge sends the loop back for one bounded extra
+	// round (the announce-then-stop undelivered-artifact check). nil = runs
+	// finalize unguarded. See FinalGuard in final_guard.go.
+	FinalGuard FinalGuard
 }
 
 // SkillPackActivator resolves an agent's subscribed skill-pack names for a run