Log "skipped, inputs unchanged" for toplevel tasks with no changes

When a toplevel task detects no input changes (local cache hit), it
previously exited silently. This adds an info log message so agents
and users can distinguish a cache hit from an empty/broken run.
The message only appears for toplevel tasks to avoid noise from subtasks.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Author: zyla

CLAUDE.md

@@ -63,6 +63,7 @@ stack test --test-arguments --accept
   - `# no toplevel` - don't wrap in taskrunner
   - `# s3` - enable S3 testing
   - `# github keys` - provide GitHub credentials
+- When testing toplevel vs subtask behavior, use real nesting (default toplevel wrapping + inner `taskrunner` calls) rather than manually setting internal env vars like `_taskrunner_build_id`
 
 ## Key Commands for Development
 

src/App.hs

@@ -186,6 +186,9 @@ main = do
 
     m_hashToSave <- readIORef appState.hashToSaveRef
 
+    when (skipped && isNothing m_hashToSave && appState.isToplevel) do
+      logInfo appState "skipped, inputs unchanged"
+
     -- Only be chatty about exit status if we were chatty about starting the work, i.e. if it is cacheable.
     when (not skipped && isJust m_hashToSave) do
       if exitCode == ExitSuccess then

test/t/file-input-unchanged-force.out

@@ -7,3 +7,4 @@ should compute again:
 [mytask] stdout | Expensive computation
 [mytask] info | success
 should not compute again:
+[mytask] info | skipped, inputs unchanged

test/t/file-input-unchanged.out

@@ -2,3 +2,4 @@
 [mytask] info | Inputs changed, running task
 [mytask] stdout | Expensive computation
 [mytask] info | success
+[mytask] info | skipped, inputs unchanged

test/t/file-input-with-spaces-unchanged.out

@@ -2,3 +2,4 @@
 [mytask] info | Inputs changed, running task
 [mytask] stdout | Expensive computation
 [mytask] info | success
+[mytask] info | skipped, inputs unchanged

test/t/raw-input-unchanged.out

@@ -2,3 +2,4 @@
 [mytask] info | Inputs changed, running task
 [mytask] stdout | Expensive computation, input is: lol
 [mytask] info | success
+[mytask] info | skipped, inputs unchanged

test/t/subtask-skip-no-message.out

@@ -0,0 +1,4 @@
+-- output:
+[mytask] info | Inputs changed, running task
+[mytask] stdout | Running task
+[mytask] info | success

test/t/subtask-skip-no-message.txt

@@ -0,0 +1,13 @@
+echo lol > input.txt
+git init -q
+git add input.txt
+
+go() {
+  taskrunner -n mytask bash -e -c '
+    snapshot input.txt
+    echo "Running task"
+  '
+}
+
+go
+go

test/t/toplevel-skip-message.out

@@ -0,0 +1,5 @@
+-- output:
+[mytask] info | Inputs changed, running task
+[mytask] stdout | Running task
+[mytask] info | success
+[mytask] info | skipped, inputs unchanged

test/t/toplevel-skip-message.txt

@@ -0,0 +1,13 @@
+# no toplevel
+echo lol > input.txt
+git init -q
+git add input.txt
+
+go() {
+  taskrunner -n mytask bash -e -c '
+    snapshot input.txt
+    echo "Running task"
+  '
+}
+go
+go