hive-runtime: compact ACP sessions through the agent's compact command

An ACP agent's session is now compacted like a claude one: proactively
once a turn crosses the percent-of-window watermark, and on the
operator's /compact or the agent's compact tool. Before, the ACP
backend's compact returned Unsupported and no watermark applied to it.

- AcpRuntime takes the same CompactionPolicy as ClaudeRuntime;
  make_session builds one PercentPolicy (with CHECKPOINT_PROMPT) and hands
  it to whichever backend runs.
- The runtime keeps the commands each session advertises in
  available_commands_update. If `compact` is among them, compaction sends
  the prompt `/compact` on the same session, which is how the ACP spec
  runs an advertised command. A proactive compaction runs the checkpoint
  turn first, as InfiniteSession does.
- With no compact command, the checkpoint turn runs, the session is
  archived, and the next turn starts a new one with the system prompt.
- Error::Unsupported had no producer left, so it and drive_turn's
  "/compact skipped" arm are gone.

Refs #4391

hive-runtime/src/lib.rs

@@ -44,8 +44,8 @@ pub trait Runtime {
         sink: &impl Sink,
     ) -> impl Future<Output = Result<Progress>>;
 
-    /// Compact the durable session now. A backend that cannot returns
-    /// [`Error::Unsupported`].
+    /// Compact the durable session now. With no session there is nothing to
+    /// compact, and this succeeds without doing anything.
     fn compact(&self, config: &Config, sink: &impl Sink) -> impl Future<Output = Result<()>>;
 
     /// Set the durable session aside so the next turn starts a fresh one.
@@ -63,7 +63,7 @@ pub trait Runtime {
 /// [`RuntimeSpec`].
 pub enum AgentRuntime<P: CompactionPolicy> {
     Claude(ClaudeRuntime<P>),
-    Acp(Box<AcpRuntime>),
+    Acp(Box<AcpRuntime<P>>),
 }
 
 impl<P: CompactionPolicy> Runtime for AgentRuntime<P> {
@@ -106,9 +106,6 @@ pub enum Error {
     /// From the ACP backend.
     #[error(transparent)]
     Acp(#[from] AcpError),
-    /// The backend does not implement this operation.
-    #[error("{0} is not supported by this runtime")]
-    Unsupported(&'static str),
 }
 
 /// Result alias for this crate.