feat(board): give the plan a paste= line the operator can actually use

--plan shipped with dir= and command= as separate fields. That is right for a
driver, which moves the pane itself and then types the command, and unusable
for a person: the first operator to read a block could not act on it at all.

Joining an absolute path from one field with a command from another is not a
saved line of output. It is the step where a session gets started in the wrong
repository - the same failure the absolute dir= exists to prevent, moved from
the machine to the human.

paste= is the two already joined, cd <dir> && <command>, one thing to select.
Both forms stay: the two consumers want the same fact shaped differently, and
neither shape serves both.

Emitted only alongside command=, never once per block. A `paste=cd X && ` with
nothing after it would run the cd and then a bare newline, which fails
SILENTLY - the operator ends up in the right directory with no session started
and no error to notice.

board-selftest: 86 -> 89 checks.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_011eKRzhD1RqY2MxvDEkrJAM

package.json

@@ -1,6 +1,6 @@
 {
   "name": "repo-mailbox",
-  "version": "0.16.0",
+  "version": "0.17.0",
   "private": true,
   "type": "module",
   "engines": {