install.sh: point at construct doctor after a fresh install

[edwin-zvs]

One line on the installer's closing message:

Done. Try:  construct
If anything looks off:  construct doctor

Why here

A just-installed machine is the likeliest one to be missing a harness CLI, an
OAuth login, or a writable state directory — which is precisely what doctor
checks. Before this, the only way to discover the command existed was
construct --help, which is the wrong surface for something whose entire job
is to be findable at the moment something is already wrong.

The installer also hand-rolls its own PATH check a few lines up:

case ":${PATH}:" in
  *":${BIN_DIR}:"*) ;;
  *) say "⚠  ${BIN_DIR} is not on your PATH. …"

That's a weaker version of doctor's env.path finding — a case on $PATH
can't see a second construct shadowing the one the installer just wrote.

Why it's safe to suggest at this moment

Doctor never starts a daemon, and a fresh install — no config file, no daemon
running, no sessions — reports as healthy and exits 0. Verified against an
isolated home: 26 ok, 10 info, 5 warn, 0 error — construct looks healthy.
New users won't be greeted by a wall of warnings for having just installed.

Not changed

• construct upgrade — its output is already more specific than a doctor
  pointer would be. The one state where doctor is relevant post-upgrade is
  client/daemon build skew, and upgrade already detects that and prints three
  concrete ways to fix it. Redirecting to doctor there would be strictly
  worse.
• README.md — worth a mention in §1 Requirements (that list maps one-to-one
  onto doctor's harnesses and logins sections), but out of scope for this PR.

sh -n install.sh passes.