disk_watch: stay silent when the agent owns nothing worth deleting

The todo fired on filesystem usage alone, so every agent on a busy host
got woken by a disk it had no power over. Measured case: a 927G volume
at 81%, 742G used, of which one agent's entire tree was 5.3G. The todo
cost that agent a full turn to arrive at "not actionable".

`summary_for` now also requires a non-empty `big_dirs()` result. An
agent that owns no oversized directory cannot free meaningful space, so
there is nothing to say to it; the shared store filling up is the host's
signal. Agents that ARE sitting on a stale `target/` still get told, with
the same bucketed anti-nag summary as before.

Drops the conditional around the "Biggest directories" section, which is
now unreachable when empty. Existing tests passed `&[]` as a
don't-care — they now pass a directory, since that argument became
load-bearing.

Closes #2759

hive-agent/src/disk_watch.rs

@@ -115,8 +115,14 @@ fn probe() -> Option<String> {
 /// percentage + set of oversized directories. Separated so the threshold
 /// and — more importantly — the *stability* of the summary are unit-tested
 /// without a real filesystem.
+///
+/// Silent unless the agent owns something worth deleting. A full disk the
+/// agent did not fill is not its problem to solve: it cannot free host
+/// bytes, so the todo would only ever cost a turn to conclude "not
+/// actionable". The shared store filling up is the host's signal, not an
+/// agent's.
 fn summary_for(pct: u64, dirs: &[PathBuf]) -> Option<String> {
-    if pct < WARN_PCT {
+    if pct < WARN_PCT || dirs.is_empty() {
         return None;
     }
     let bucket = pct / BUCKET_PCT * BUCKET_PCT;
@@ -125,16 +131,12 @@ fn summary_for(pct: u64, dirs: &[PathBuf]) -> Option<String> {
          Only delete things that are actually big (tens of GiB); a few MB of notes won't move the needle.\n\
          First candidates: regenerable build output in your workspace (`target/`, `node_modules/`, `dist/`).\n\
          Never delete anything still needed. If nothing is safe to drop, tell the operator you need more space \
-         rather than forcing it — be cautious with deletions in general."
+         rather than forcing it — be cautious with deletions in general.\n\
+         Biggest directories under your own tree (`du -sh` them before removing anything):"
     );
-    if !dirs.is_empty() {
-        out.push_str(
-            "\nBiggest directories under your own tree (`du -sh` them before removing anything):",
-        );
-        for dir in dirs {
-            // Infallible: writing into a String.
-            let _ = write!(out, "\n  - {}", dir.display());
-        }
+    for dir in dirs {
+        // Infallible: writing into a String.
+        let _ = write!(out, "\n  - {}", dir.display());
     }
     Some(out)
 }
@@ -273,15 +275,23 @@ mod tests {
 
     #[test]
     fn under_threshold_is_silent() {
-        assert!(summary_for(WARN_PCT - 1, &[]).is_none());
+        assert!(summary_for(WARN_PCT - 1, &[p("/agents/a/state/hh/target")]).is_none());
     }
 
     #[test]
     fn at_threshold_speaks_up() {
-        let s = summary_for(WARN_PCT, &[]).expect("todo");
+        let s = summary_for(WARN_PCT, &[p("/agents/a/state/hh/target")]).expect("todo");
         assert!(s.contains("over 80% full"));
         assert!(s.contains("tens of GiB"));
         assert!(s.contains("tell the operator"));
+        assert!(s.contains("/agents/a/state/hh/target"));
+    }
+
+    /// A full disk the agent did not fill is not its problem: it cannot
+    /// free host bytes, so naming no directories means saying nothing.
+    #[test]
+    fn full_disk_with_nothing_big_of_ours_is_silent() {
+        assert!(summary_for(99, &[]).is_none());
     }
 
     /// The anti-nag property: usage drifting inside one bucket must render
@@ -289,15 +299,17 @@ mod tests {
     /// nothing wakes the agent.
     #[test]
     fn drift_inside_a_bucket_is_identical() {
-        let a = summary_for(85, &[]).expect("todo");
-        let b = summary_for(89, &[]).expect("todo");
+        let dirs = [p("/agents/a/state/hh/target")];
+        let a = summary_for(85, &dirs).expect("todo");
+        let b = summary_for(89, &dirs).expect("todo");
         assert_eq!(a, b);
     }
 
     #[test]
     fn crossing_a_bucket_changes_the_summary() {
-        let a = summary_for(89, &[]).expect("todo");
-        let b = summary_for(90, &[]).expect("todo");
+        let dirs = [p("/agents/a/state/hh/target")];
+        let a = summary_for(89, &dirs).expect("todo");
+        let b = summary_for(90, &dirs).expect("todo");
         assert_ne!(a, b);
         assert!(b.contains("over 90% full"));
     }
@@ -353,7 +365,7 @@ mod tests {
         let dir = tempfile::tempdir().expect("tempdir");
         let todos = Todos::open(&dir.path().join("state.sqlite")).expect("open");
         let wake = Notify::new();
-        let summary = summary_for(90, &[]).expect("todo");
+        let summary = summary_for(90, &[p("/agents/a/state/hh/target")]).expect("todo");
 
         reconcile(&todos, &wake, Some(&summary));
         assert_eq!(todos.list(Some(SUBSYSTEM)).expect("list").len(), 1);