Deprecate public workflows visibility model

[migmartri]

Context

The Workflow.public boolean field and the "public workflow → publicly discoverable" visibility model is being phased out in favor of RBAC-driven visibility (per-project visibility under each org).

Today, several authorization paths short-circuit on public-workflow membership:

• pkg/biz/referrer.go: StoredReferrer.InPublicWorkflow field, hydrated in pkg/data/referrer.go::hydrateWorkflowsInfo
• pkg/data/referrer.go::isReferrerVisible: short-circuits to true when the referrer is in any public workflow, bypassing RBAC
• pkg/data/referrer.go: multiple workflow.Public(true) / workflow.Public(*public) predicates in doGet and the project-scoped attestation digest query
• pkg/data/casmapping.go::154: return workflow.Public, nil
• pkg/data/workflow.go::345: wfQuery.Where(workflow.Public(*opts.WorkflowPublic))
• pkg/biz/workflowrun.go::727: authorization bypass when wfRun.Workflow.Public
• API filters: --public / WorkflowPublic filter throughout the proto + service layers

Scope of removal

1. Remove the public-workflow visibility short-circuit from isReferrerVisible — visibility becomes RBAC-only.
2. Stop using the workflow.Public(...) ent predicate in new code; replace existing callsites with RBAC-derived predicates or remove the filter.
3. Eventually drop the public field from the Workflow schema (separate migration step — keep the column around during transition for rollback).
4. Remove the proto-level public-workflow filter knobs that are no longer meaningful under RBAC.

Related

Surfaced while working on the referrer discovery project/version filter (#3155, PR #3158). That PR explicitly does not change public-workflow behavior — it adds RBAC-aware project filtering alongside the existing public short-circuit. Once this deprecation lands, the public branch in the referrer project-scope subquery can be removed.

Acceptance criteria

• workflow.Public predicate removed from all production code paths (only schema + migration retain knowledge of the column)
• StoredReferrer.InPublicWorkflow removed and dependent visibility logic updated to RBAC-only
• Public-workflow API filters deprecated and eventually removed
• Test suite updated to cover RBAC-only visibility (no test depends on the public short-circuit)