CLI: changeset commands default to JARL_CHANGESET, which is checked first

jarl changeset show/diff/apply/discard take the changeset from -C or
JARL_CHANGESET when no ID is given, and remove takes [ID] SEQ. A changeset
from JARL_CHANGESET is checked before anything uses it (staging included):
it must exist, be open and be the caller's own, so a stale export can't
stage into an applied changeset or into another user's draft that later
got its ID. Commands say on stderr which changeset they use, and apply and
discard remind to unset JARL_CHANGESET. -C and ID arguments are used as
given.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

CHANGELOG.md

@@ -25,6 +25,15 @@ All notable changes to Jarl are documented here. The format follows
 
 ### Added
 
+- `jarl changeset show`, `diff`, `apply`, `discard` and `remove` take the
+  changeset from `-C` or `JARL_CHANGESET` when no ID is given
+  (`jarl changeset apply`). A changeset from `JARL_CHANGESET` is now
+  checked before it is used, for staging too: it must exist, be open and
+  be yours, so a stale export (an applied changeset, or an ID that later
+  turned into someone else's web UI draft) can't silently take your
+  changes; the error says what to do. Commands say on stderr which
+  changeset from `JARL_CHANGESET` they use, and `apply`/`discard` remind
+  you to `unset JARL_CHANGESET`. `-C` and ID arguments are used as given.
 - Address ranges such as `10.0.10.101-10.0.10.199` or
   `2001:db8::100-2001:db8::1ff` (also `!` negated, and with spaces around
   the dash) wherever an address match list is used: ACLs, views'