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).

examples/echo-agent/README.md

@@ -31,4 +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 under `examples/echo-agent/.sessions/` (per-cwd subdirectory, one `.jsonl` log per session). Clean up with: `rm -rf examples/echo-agent/.sessions`
+The session is persisted under `.sessions/` relative to the directory you launch the demo from. `pnpm run demo:echo` runs from the repo root, so the logs land in `<repo-root>/.sessions/` (a session with no cwd goes in the `_no-cwd/` bucket, one `.jsonl` log per session). Clean up with: `rm -rf .sessions`