Add calcalist status, and make routing markers discoverable

The marker that steers a new event is the endpoint's id, which until now meant
remembering what was written in a TOML file while typing into a calendar app.

status lists the endpoints, what each aggregate publishes and mirrors, where
untagged events go, and the exact @markers that would work — read-only sources
omitted, since offering one as a destination would mislead.

A refused marker now names the alternatives rather than only reporting that it
failed, and matching ignores case: the name is typed by hand, and capitalisation
is not worth failing over.

133 tests.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

TODO.md

@@ -58,6 +58,9 @@ integration tests, which are listed with the remaining gaps at the end.
 - [x] `webcal` URLs may come from a command, for feeds whose address is itself a
       credential — Google's secret iCal address being the case in point
 - [x] `doctor` validates the generated pimsync config and each Google authorisation
+- [x] `calcalist status` — endpoints, aggregates, how many events are mirrored, and the
+      exact `@marker` names an event can carry, so routing is discoverable without
+      opening the config file
 
 ### Tests