fix(session): synthesize tool results for interrupted tool calls on crash recovery (review #33)

The loop logs the assistant/message (carrying tool-call blocks) BEFORE running
the tools, so a crash mid-tool leaves durable tool calls with no matching
tool/result. interruptedTurnClosers only added step/end + turn/end, so a resumed
session's deriveMessages() replayed a dangling assistant tool-call — which every
provider rejects as an invalid transcript on the next request.

interruptedTurnClosers now scans the interrupted turn for tool-call blocks
without a matching tool/result and synthesizes an error tool/result for each
(before the step/end), so the rehydrated history is a valid transcript. Adds a
dedicated repair.spec.ts and a shared-contract case proving both backends pair
every orphaned call with a result. Docs (ADR 0018, both persistence READMEs,
load() JSDoc) updated.

Also fixes the echo-agent README session-cleanup path: demo:echo runs from the
repo root, so sessions land in <repo-root>/.sessions/_no-cwd/, not
examples/echo-agent/.sessions/ (review #33).

packages/session-persistence-jsonl/README.md

@@ -24,7 +24,7 @@ The JSONL durable session-persistence backend — a concrete `SessionPersistence
 
 - **Lazy materialization.** `create(meta)` writes nothing; the `.jsonl` (header + first batch) is written atomically (temp-write + `fsync` + rename) on the first `append`. A created-but-never-appended session leaves nothing on disk and is absent from `has`/`list`.
 - **Append-only.** Committed events (at or below a flushed `turn/end`) are never rewritten. Subsequent appends are line appends at EOF + `fsync`.
-- **Crash recovery — close, don't truncate.** A crash can leave a log whose final turn never closed (real events after the last `turn/end`). `load` PRESERVES those events (a turn can be huge — they are real work) and closes the orphaned turn by durably appending synthetic boundary events (a `step/end` if a step was open, then `turn/end {kind:'interrupted'}`), returning a balanced log. Only a never-fully-written **torn tail fragment** (a final line with no newline / unparseable) is `ftruncate`d away before the closers are written. See ADR 0018.
+- **Crash recovery — close, don't truncate.** A crash can leave a log whose final turn never closed (real events after the last `turn/end`). `load` PRESERVES those events (a turn can be huge — they are real work) and closes the orphaned turn by durably appending synthetic boundary events: an error `tool/result` for every `tool-call` the crash left unanswered (the loop logs the assistant message before running the tools, so a mid-tool crash leaves dangling calls — and `deriveMessages()` would replay an assistant tool-call with no result, which providers reject), then a `step/end` if a step was open, then `turn/end {kind:'interrupted'}`, returning a balanced log. Only a never-fully-written **torn tail fragment** (a final line with no newline / unparseable) is `ftruncate`d away before the closers are written. See ADR 0018.
 - **Contiguous-seq.** `load` rejects a mid-log parse error or `seq` gap (unloadable); `append` rejects a batch whose first `seq` does not continue the stored log, and rejects non-JSON-serializable `event.data` naming the offending event type.
 - **Format version.** Only v1 is supported; `load` rejects an unknown version. A future format change requires a version bump + migration.