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.

examples/coding-agent/src/session-jsonl.ts

@@ -1,36 +0,0 @@
-import { appendFile } from 'node:fs/promises'
-import { join } from 'node:path'
-import type { Context } from 'cordis'
-import type { Session, SessionEvent } from '@deepseek-ai/dsh-session'
-
-export const name = 'session-jsonl'
-export const inject = ['sessions']
-
-/**
- * Minimal persistence plugin: buffers session events (write-behind) and
- * drains to a JSONL file at every `session/flush` checkpoint — the pattern a
- * real JSONL/sqlite persistence plugin would follow.
- */
-export function apply(ctx: Context) {
-  const buffers = new Map<Session, SessionEvent[]>()
-  const path = (session: Session) => join(import.meta.dirname, '..', `${session.id}.jsonl`)
-
-  ctx.on('session/event', (session, event) => {
-    let buffer = buffers.get(session)
-    if (!buffer) buffers.set(session, buffer = [])
-    buffer.push(event)
-  })
-
-  const flush = async (session: Session) => {
-    const buffer = buffers.get(session)
-    if (!buffer?.length) return
-    const lines = buffer.splice(0).map(event => JSON.stringify(event) + '\n').join('')
-    await appendFile(path(session), lines)
-  }
-
-  ctx.on('session/flush', flush)
-  ctx.effect(() => () => {
-    // drain remaining buffers on dispose
-    for (const session of buffers.keys()) void flush(session)
-  }, 'session-jsonl')
-}