Four more friendly verbs, all pure compositions of the base 9P
operations against the existing namespace (no server change):
- tag <doc> <id> <tag> [--remove]: toggle a single tag, mirroring
org-toggle-tag, instead of rewriting the whole tag list.
- prop <doc> <id> <key> [value] [--delete]: get, set, or delete one
property, mirroring org-set-property, instead of the whole drawer.
- todo <doc> <id> [state]: set the keyword, or with no argument cycle
to the next state following the /keywords sequence, like org-todo.
- tree <doc>: print the recursive section outline.
Each edit reads the relevant namespace file, modifies it, and writes it
back; all validate the section exists first for a clear error.
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