feat(session-persistence): abstract seam + JSONL backend + wiring

Add the durable session-persistence capability seam (ADR 0016): an
abstract SessionPersistence service (dsh-session-persistence,
ctx.sessionPersistence) defining create/append/load/list/has/delete/
update over the existing SessionEvent — no parallel persisted type — and
a first implementation (dsh-session-persistence-jsonl): an append-only
JSONL log per session with crash-safe atomic writes, truncation-repair
of a never-committed crash tail, and a read/replay path. SessionMeta
(format version, cwd, lineage) travels out-of-log via session.header.

A shared runPersistenceContract suite holds every backend to the same
append-only / contiguous-seq / lazy-materialization / serializability
semantics.

Config-driven create() now uses a per-run ${id}-session-<uuid> session
id so a fixed name no longer collides with an on-disk log once a durable
backend is loaded; each run is a new session (a demo simplification). The
examples drop their hand-rolled session-jsonl.ts and load the JSONL
backend via cordis.yml; CI smoke-loads it too.

The agent-facing create/resume factory that consumes load() is a
separate seam, deferred to a follow-up; this change stops at the load
primitive and does not reach into the loop.

.gitignore

@@ -6,6 +6,8 @@ lib/
 .yarn/
 yarn-error.log
 examples/*/*.jsonl
+.sessions/
+examples/*/.sessions/
 coverage/
 .doc-typecheck-*/
 .vscode/