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/echo-agent/README.md

@@ -7,7 +7,7 @@ Runnable demo: stdin chat with a scripted mock model and an echo tool.
 - A complete Cordis app loaded from `cordis.yml` — the standard "stack of plugins" pattern
 - `mock-llm.ts` — a mock `LlmAdapter` that streams scripted responses and calls the `echo` tool when the user types "echo <something>"
 - `echo-tool.ts` — a tool registered via `ctx.tools.register()` that echoes text back uppercased
-- `session-jsonl.ts` — a minimal persistence plugin: write-behind buffering of `session/event` notifications, drained to a JSONL file at `session/flush`
+- `@deepseek-ai/dsh-session-persistence-jsonl` — the durable JSONL persistence backend (loaded from `cordis.yml`, `root: ./.sessions`): append-only event log per session with crash-safe atomic writes, replacing the old write-only example plugin
 - `stdio-chat.ts` — a minimal UI plugin: reads stdin lines and `send`/`steer`s the agent, renders stream deltas, tool calls, and tool results
 
 ## Plugin files
@@ -16,10 +16,11 @@ Runnable demo: stdin chat with a scripted mock model and an echo tool.
 |---|---|---|
 | `mock-llm.ts` | `LlmAdapter` registration | `ctx.llm.registerAdapter(['mock-echo'], …)`, streaming chunks with proper `block-start`/`block-end` protocol |
 | `echo-tool.ts` | Tool registration | `ctx.tools.register(defineTool(…))` with typed `execute` args, tool execution returning `ContentBlock[]` |
-| `session-jsonl.ts` | Persistence | `session/event` listener + `session/flush` drain, fiber-dispose cleanup |
 | `stdio-chat.ts` | UI | `agent/stream-chunk`, `session/event` (tool/*), stdin→send/steer |
 | `start.ts` | Bootstrap | `Context` + `Loader` + `plugin-include` wired to `cordis.yml` |
 
+Persistence is the shared `@deepseek-ai/dsh-session-persistence-jsonl` plugin (not a per-example file).
+
 ## Run
 
 ```sh
@@ -30,4 +31,4 @@ node --expose-internals --import tsx examples/echo-agent/start.ts
 
 Type a message and press Enter. "echo <text>" triggers a tool call round-trip (the mock model requests the `echo` tool, which echoes the text uppercased, and the next model step acknowledges it).
 
-The session is persisted to `<session-id>.jsonl` in the `examples/echo-agent/` directory. Clean up with: `rm -f examples/echo-agent/*.jsonl`
+The session is persisted under `examples/echo-agent/.sessions/` (per-cwd subdirectory, one `.jsonl` log per session). Clean up with: `rm -rf examples/echo-agent/.sessions`

examples/echo-agent/cordis.yml

@@ -46,8 +46,10 @@
 - id: echo-tool
   name: './src/echo-tool.ts'
 
-- id: session-jsonl
-  name: './src/session-jsonl.ts'
+- id: session-persistence
+  name: '@deepseek-ai/dsh-session-persistence-jsonl'
+  config:
+    root: './.sessions'
 
 - id: stdio-chat
   name: './src/stdio-chat.ts'

examples/echo-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')
-}