v2 surface migration plan: retire frozen ledger-named machine surfaces

[MaxGhenis]

The 2026-08-07 rename (PR #142) deliberately froze every machine surface at its v1 ledger-named form. This issue scopes the deliberate v2 migration:

• Schema ids: ledger.bundle*.v1, ledger.build_artifact.v1, ledger.consumer_fact.v1, ledger.relational.v1, policyengine_ledger.{consumer_artifact,target_profile,resolved_target,approved_agents}.v1
• Fact-identity hash domains (ledger.*.v2) — renaming changes every fact hash; needs an explicit identity-migration story (dual-hash window or v3 domains) coordinated with the witnessed journal
• R2 buckets ledger-raw/ledger-derived and storage.r2 manifest keys; ledger.db artifact filename
• Supabase schema "ledger", mirror tables/columns (ledger_builds, ledger_source_status)
• Governance role ids (ledger-*) and concept authorities (ledger-us)
• POLICYENGINE_LEDGER_* / LEDGER_* env names (dual-read fallback)
• CLI ledger alias retirement after consumers migrate (María's publish flow, microcosm build scripts)
• codex/thesis-ledger-facts branch + thesis witness machinery (Thesis session owns the canonical-URL PR)
• thesisinstitute.org /ledger app dir + journal data paths (deferred with the witness machinery)