Editing a task's canonical-context doc body previously meant looking up
its `canonical_context_id` (e.g. via `heph list --json`) and then
`heph node update <doc-id> --body`. Add a `heph context <task-id>`
command that resolves the canonical-context doc from the task's outgoing
links and:
* prints the body with no flag,
* `--body <text>` replaces it (`-` reads stdin, matching `node update`),
* `--append <text>` adds a blank-line-separated paragraph.
Errors clearly when the id has no canonical-context doc (e.g. a plain
doc node rather than a task). Purely a client-side CLI convenience — no
new RPC.
Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
