feat(todo): add numbered tasks, edit, done, and future filtering

Implement smoother todo editing with the following features:
- Display numbered tasks [1], [2], etc. in `streamd todo`
- Add `streamd todo N edit` to open editor at task line
- Add `streamd todo N done` to insert @Done after @Task
- Add `--show-future` flag to include future tasks (hidden by default)

src/error.rs

@@ -16,6 +16,21 @@ pub enum StreamdError {
 
     #[error("TOML error: {0}")]
     TomlError(#[from] toml::de::Error),
+
+    #[error("Invalid task number {0}: only {1} tasks available")]
+    InvalidTaskNumber(usize, usize),
+
+    #[error("Task shard missing file path")]
+    MissingFilePath,
+
+    #[error("Invalid line number in task")]
+    InvalidLineNumber,
+
+    #[error("Multiple @Task markers found in {0}:{1} - cannot auto-insert @Done")]
+    MultipleTaskMarkers(String, usize),
+
+    #[error("No @Task marker found in {0}:{1}")]
+    NoTaskMarker(String, usize),
 }
 
 impl From<StreamdError> for miette::Report {