fix(watcher): copy files instead of moving them, leave originals for Obsidian

Files dropped into brain/raw/ are now copied to processed/ or failed/ rather
than moved. A .processed or .failed marker is written next to the original so
the watcher skips it on subsequent polls without deleting it. This keeps
Syncthing-synced Obsidian vaults intact after ingestion.

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

ingestion/internal/watcher/watcher_test.go

@@ -81,11 +81,15 @@ func TestStart_ProcessesFile(t *testing.T) {
 		}
 		time.Sleep(20 * time.Millisecond)
 	}
-	require.True(t, found, "file should be moved to processed/")
+	require.True(t, found, "file should be copied to processed/")
 
-	// Original file should be gone.
+	// Original file should still exist (copy, not move — keeps Obsidian vault intact).
 	_, err := os.Stat(rawFile)
-	assert.True(t, os.IsNotExist(err), "original file should be gone from raw/")
+	assert.NoError(t, err, "original file should remain in raw/")
+
+	// A .processed marker should exist next to the original.
+	_, err = os.Stat(rawFile + ".processed")
+	assert.NoError(t, err, ".processed marker should be written")
 
 	// Wiki page should exist.
 	wikiPath := filepath.Join(brainDir, "wiki", "sources", "shape-up-book.md")
@@ -130,11 +134,15 @@ func TestStart_MovesToFailedOnError(t *testing.T) {
 		}
 		time.Sleep(20 * time.Millisecond)
 	}
-	require.True(t, found, "file should be moved to failed/")
+	require.True(t, found, "file should be copied to failed/")
 
-	// Original file should be gone from raw/.
+	// Original file should still exist (copy, not move — keeps Obsidian vault intact).
 	_, err := os.Stat(rawFile)
-	assert.True(t, os.IsNotExist(err), "original file should be gone from raw/")
+	assert.NoError(t, err, "original file should remain in raw/")
+
+	// A .failed marker should exist next to the original.
+	_, err = os.Stat(rawFile + ".failed")
+	assert.NoError(t, err, ".failed marker should be written")
 
 	// log.md should contain a watcher error entry.
 	logContent, err := os.ReadFile(filepath.Join(brainDir, "log.md"))