New `heph context <task-id>` command reads or edits a task's canonical-context doc body **by task id**, with no manual `canonical_context_id` lookup. With no flag it prints the body; `--body <text>` replaces it (`-` reads stdin, like `node update`); `--append <text>` adds a blank-line-separated paragraph. Errors clearly on a node that has no canonical-context doc (e.g. a plain doc, not a task).