04.2 — Read-side shadow, cutover, retirement

Import phases 2–4 from ../../tinyagents-session-migration-design.md.

Steps

Shadow (phase 2): add a store-backed reader implementing the same surface as transcript.rs (read_transcript, history load in session_io.rs, .md render). Behind a flag, read BOTH and log divergence (grep-friendly [session_import] prefix); run the 11-fixture matrix + live dogfood.
Cutover (phase 3): run openhuman.session_import_run automatically once at boot (idempotent marker migrations/session_import_v1 already exists), flip default reads to the store, keep legacy JSONL as write-through backup for one release.
Retire (phase 4): stop legacy writes; delete the legacy stack.
Sub-agent transcript stems (__ chains) and worker-thread mirrors must resolve through store descriptors — verify against subagent_runner mirroring and agent_orchestration/subagent_sessions/ (that store also folds into descriptors here).
Parser retirement gate: keep agent/dispatcher.rs and harness/parse.rs until no live provider path needs XML/P-format prompt dialects, native text-fallback parsing, TinyAgents text-mode response parsing, or sub-agent checkpoint cleanup.

session/transcript.rs (1347) + transcript_tests.rs.
session/migration.rs (373) + tests — old date-folder migration, still invoked at startup through the migrate_session_layout_if_needed re-export until the read-side cutover has a replacement startup gate.
session/turn/session_io.rs (391) — replaced by store reader/writer.
session_db/{ops,store,schemas,types}.rs generic session parts (keep run_ledger/ until 05).
agent_orchestration/subagent_sessions/ (~650).
src/openhuman/session_import/ (whole domain + RPC controller from core/all.rs) once a marker-versioned release has shipped.

All fixtures byte/render-identical between readers before flip.
Old UI session lists still answer by OpenHuman session key (compat projection over descriptors).