fix(acp): address review of the terminal-card alignment (exit parse, background/error, capability snapshot)

Codex + an independent review pass found three real defects in the prior commit:

1. parseExitStatus could misreport a SUCCESSFUL command as a failure: a clean
   exit 0 appends no marker, so output ending in "[exit code: 5]" (no trailing
   newline) was read as the marker. Anchor the parse to a LEADING newline —
   renderResult always inserts one before a real marker, so a body that merely
   ends in marker-like text no longer matches. A narrow residual (a clean exit 0
   whose final line is exactly the marker) is inherent to the replay-only-sees-
   text design and documented; the complete fix (a structured exit on the event)
   is the RFC's named escape hatch.

2. A run_in_background start and an isError result were rendered as exited
   terminal cards with a false exit-0 pill. A background start returns a task-id
   ack (not a streamed terminal) and is no longer marked terminal; an isError
   result (spawn failure / abort) carries no exit pill.

3. The terminal capability was re-read live on the result path, so a second
   initialize between a call and its result could desync them (orphan
   terminal_output or clobbered card). Snapshot the capability per session at
   creation (SessionRecord.terminalEnabled) so call and result always agree.

Also reword the reference-parity claim: keeping the description as a content
block in terminal mode is a DELIBERATE divergence (claude-agent-acp drops it).
Tests added for each; with-key e2e still green.

docs/rfc/implemented/2026-06-18-acp-terminal-and-tool-rendering.md

@@ -6,7 +6,7 @@ Status: implemented
 
 The ACP bridge lets each tool own its call rendering via `presentCall`/`presentResult` (see [tool-call UI presentation](../proposed/2026-06-14-acp-agent-client-protocol.md) and `packages/tools`). For `bash` we surface the exact command as the `tool_call` title, the model's `description` as a content text block, `kind: 'execute'`, and the completed output wrapped in a fenced ` ```console ` text block.
 
-That is a correct, capability-free baseline, but not how the reference editors render a *terminal* tool at its best. An editor like Zed has a dedicated terminal tool-call card — a header showing the working directory, the command as the label, the command output rendered as a terminal, and an exit-status pill — but it only builds that card when the `tool_call` carries terminal metadata (below). With a plain text block the output appears as static markdown and there is no cwd header. (Zed also HIDES `rawInput` for `kind: 'execute'`, which is why the command IS the title — both reference adapters do the same — and the human-readable description rides as a separate content block, since a terminal card has no description slot.)
+That is a correct, capability-free baseline, but not how the reference editors render a *terminal* tool at its best. An editor like Zed has a dedicated terminal tool-call card — a header showing the working directory, the command as the label, the command output rendered as a terminal, and an exit-status pill — but it only builds that card when the `tool_call` carries terminal metadata (below). With a plain text block the output appears as static markdown and there is no cwd header. (Zed also HIDES `rawInput` for `kind: 'execute'`, which is why the command IS the title — both reference adapters do the same. The human-readable description rides as a separate content block above the card; note this is a DELIBERATE divergence — claude-agent-acp DROPS the description in terminal mode and renders only the card — we keep the summary visible alongside.)
 
 ## Key finding: agent-executed terminals use a `_meta` convention, NOT `terminal/create`