say a missing opencode binary out loud instead of hanging

Bun.spawn throws on a missing or non-executable binary rather than resolving to a failed
process, and that throw escaped runOpenCodeTurn entirely — past the bookkeeping, out of the
sidecar command handler, with no opencode:event ever emitted. The browser sat on a spinner
nothing could end, because the code that ends turns had not been reached.

A wrong OPENCODE_BIN is the ordinary way to get there, so the message names the path it
tried: that is the difference between a fix and a debugging session.

Also records that messageCount is not a gap. SessionList renders an OpenCode badge in place
of the count for those rows, so the hardcoded 0 never reaches a screen, and computing a real
one would cost an HTTP call per listed session — the session record has no count field — to
populate something nothing shows.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

docs/opencode-parity.md

@@ -102,6 +102,11 @@ passed that one model.
 Until the model question is answered, turns stay on `opencode run --dir`, which is verified working on
 1.18.16.
 
+**`messageCount` is a non-issue, not a gap.** `SessionList.tsx:197-203` renders an `OpenCode` badge in
+place of the count for OpenCode rows, so the hardcoded `0` is never displayed. Computing a real count
+would cost one HTTP call per listed session — the session record carries no count field — to populate
+something nothing renders. Left alone deliberately.
+
 **Images are done, and they never needed the fork** (bucket 1 lists them as "No — see B4", and Phase 4
 put them behind the migration). `opencode run` takes attachments with `--file`, so the subprocess path
 carries them today: the sidecar spills each image to a temp file for the turn and removes it in