Adds the todo panel example under examples/extensions, updates example manifests and READMEs to match the current extension API, and surfaces extension install/load commands in zot --help.
extension commands appeared in the autocomplete popup but invoking
them produced "unknown command: /summon". The submit-handler path
already tried the extension manager before erroring, but the popup-
enter path (suggest.Selection -> runSlash) bypassed that check and
fell straight into runSlash's switch, where the default case bailed
with the generic error.
Fix: runSlash's default branch now also consults
cfg.Extensions.HasCommand and dispatches via invokeExtensionCommand
when matched. Both UI paths (typed-and-enter, popup-enter) now route
identically. Built-in cases above default still always win on
conflict.
Also adds examples/extensions/clock — a node extension demonstrating
the wire protocol from a non-Go runtime. Pure stdlib (readline +
process), no npm install. Registers /now (display) and /uptime
(prompt). Documented in its README; the protocol works the same
from any language.
