review classifier: every ACP run gets one shared, empty cwd

opencode writes the session's cwd into the system message's environment
block ("Working directory: …", "Workspace root folder: …"), ahead of the
PR's user message. Each ACP classification ran with its per-call scratch
directory, /tmp/review-classifier-<pid>-<counter>, as cwd, so every call
sent a different system message and the provider's prompt cache could not
match past it.

The session's cwd is now one fixed directory, <tmp>/review-classifier-acp-cwd,
shared by every run. HOME (the config, system.md and opencode's own state)
stays a directory per run, removed after it, as before. With no tool and
no project config nothing writes into the cwd: a live opencode 1.15.10 run
through initialize, session/new and a prompt left it empty, with its state
all under HOME. So concurrent runs can share it, and no state carries from
one run to the next.

Refs #5245

docs/swarm/review-classifier.md

@@ -124,7 +124,11 @@ resume, no tools (`--tools ""`), no MCP server (`--strict-mcp-config` and no
 config) and an empty directory of its own.
 
 On the ACP runtime, each classification is one prompt on a new `opencode acp`
-session in an empty directory of its own. The service drives it through
+session. Its config and opencode's state live in a directory of its own,
+removed after it. Its working directory is one empty directory that every
+classification shares: opencode writes that path into the system message, so
+one path keeps the system message the same from one classification to the
+next, and the provider's prompt cache can hold. The service drives it through
 [`hive-runtime`](../../hive-runtime/README.md)'s `AcpRuntime`, as the subagent
 daemon does a one-run subagent, and stops the agent when the classification
 ends. The agent has no tool. opencode runs some tools, `read` among them,