hive-runtime: record a new ACP session only once its first prompt is answered

The session id was written to the session file right after session/new,
but the system prompt rides on the first prompt only. If that prompt
failed, the next turn (or the next harness start) resumed the recorded
session as not-new and the system prompt never reached it.

The id is now written after the first session/prompt gets its reply.
A failed first prompt leaves nothing recorded, so the next turn starts
a new session and sends the system prompt again. Chosen over a separate
"system prompt delivered" marker: one file, and "recorded" already means
"usable".

Tests drive AcpRuntime against a scripted sh agent that fails the first
prompt: in-process and across a restart, the retry is a new session
carrying the system prompt.

Also: PermissionPolicy now sees a PermissionAsk (kind plus the MCP
server the tool belongs to, matched by name against the servers handed
to the session), so a caller can tell MCP tool calls from other `other`
requests.

Refs #4391

hive-runtime/Cargo.toml

@@ -14,3 +14,6 @@ serde_json.workspace = true
 thiserror.workspace = true
 tokio.workspace = true
 tracing.workspace = true
+
+[dev-dependencies]
+tempfile = "3"