docs: add todo edit/done and --show-future documentation

Document the new todo command features:
- Numbered task display
- `streamd todo N edit` for editing tasks
- `streamd todo N done` for marking tasks done
- `--show-future` flag for including future tasks
- Add R21 specification for todo command behavior

README.md

@@ -21,7 +21,10 @@ Within files, `@`-prefixed markers at the beginning of paragraphs or headings de
 ## Commands
 
 - `streamd` / `streamd new` — Create a new timestamped markdown entry, opening your editor
-- `streamd todo` — Show all open tasks (shards with `@Task` markers)
+- `streamd todo` — Show all open tasks (shards with `@Task` markers), numbered for easy reference
+- `streamd todo N edit` — Edit task N in your editor, jumping to the task's line
+- `streamd todo N done` — Mark task N as done by inserting `@Done` after `@Task`
+- `streamd todo --show-future` — Include tasks with future dates in the listing
 - `streamd edit [number]` — Edit a stream file by index (most recent first)
 - `streamd timesheet` — Generate time reports from `@Timesheet` markers
 
@@ -60,4 +63,8 @@ The timesheet command will calculate expected vs actual working hours based on t
 
 Running `streamd` opens your editor to create a new entry. After saving, the file is renamed based on its timestamp and any markers found in the content.
 
-Running `streamd todo` finds all shards marked as open tasks and displays them as rich-formatted panels in your terminal.
+Running `streamd todo` finds all shards marked as open tasks and displays them numbered in your terminal. Tasks with future dates are hidden by default (use `--show-future` to include them). Tasks are sorted by date with oldest first (task 1 is the oldest).
+
+You can quickly edit or complete tasks by number:
+- `streamd todo 1 edit` opens task 1 in your editor at the correct line
+- `streamd todo 1 done` marks task 1 as done by inserting `@Done` after `@Task`