fix(delete): referenced_by/cascade_plan omit goals citing the target

[kurosawareiji7007-hub]

What happened

referenced_by and cascade_plan walk pages, claims, and relations but never goals — even though Goal.claims / Goal.entities are first-class refs validated by storage._validate_goal_refs.

After goals landed (#427 / Goal artifact), deleting a claim (or entity) that a goal cites succeeds because the delete gate returns []. Later lifecycle.set_goal_status → update_goal → _validate_goal_refs raises ValueError: goal … references unknown claim.

Expected

referenced_by(store, "claim", id) includes goal '<gid>' when that goal cites the claim. cascade_plan / cascade approve unlinks the claim (or entity) from the goal the same way it unlinks from pages.

Repro

from vouch.storage import KBStore
from vouch import proposals, lifecycle
from vouch.models import Claim

store = KBStore.init(tmp)
src = store.put_source(b"e")
pr = proposals.propose_claim(store, text="the sky is blue", evidence=[src.id], proposed_by="a")
claim = proposals.approve(store, pr.id, approved_by="h")
gpr = proposals.propose_goal(store, title="keep sky blue", claims=[claim.id], proposed_by="a")
proposals.approve(store, gpr.id, approved_by="h")

assert proposals.referenced_by(store, "claim", claim.id)  # today: []
dpr = proposals.propose_delete(store, target_kind="claim", target_id=claim.id, proposed_by="a")
proposals.approve(store, dpr.id, approved_by="h")  # succeeds today
lifecycle.set_goal_status(store, goal_id=gpr.payload["id"] if False else store.list_goals()[0].id, status="done", actor="h")  # ValueError