fix: self-contained Homebrew launcher, node back to :build

[samyakLambda]

Real fix for #497 (the extra-Node-install complaint) — not the caveats-text
option, the shell-wrapper option. Both were on the table after the revert in
#10; this is the one I picked, with reasoning below.

Why this, not documenting the waste as intentional

rook's published npm package ships bin/rook.cjs: a small trampoline with
a #!/usr/bin/env node shebang, designed to bootstrap under whatever Node
launched it, resolve the bundled runtime via require.resolve, and re-exec
into it. Under the old, :required depends_on "node", Homebrew's own
Cleaner#rewrite_shebangs (language/node.rb) silently rewrote that
shebang to the absolute path of Homebrew's own node at build time — which
is what made the bottle work, and also exactly the extra install #497
complains about.

The trampoline's whole reason to exist is not knowing at publish time
whether a bundled runtime will be present. Homebrew's def install does
know — it just finished downloading and chmod'ing that exact binary. So
instead of relying on npm's shim plus Homebrew's shebang-rewrite trick, this
PR has def install write its own launcher directly:

#!/bin/sh
exec "<absolute path to the bundled node binary>" "<absolute path to dist/cli.js>" "$@"

Only in the success path (bundled runtime installed and verified) — the
existing opoo-and-fall-back-to-system-Node path is untouched, since npm's
original trampoline is exactly the right behavior there.

This is strictly better than either alternative on the table:

• Actually fixes #497: nothing in the installed tree has an env node
  shebang left for Homebrew to rewrite (or fail to) — depends_on "node"
  goes back to => :build, this time correctly, since nothing depends on
  the rewrite happening.
• No changes needed to the private monorepo — bin/rook.cjs is
  untouched; it's still exactly right for npm-direct/internal installs.
• Simpler signal handling than the trampoline it replaces, for this
  path: exec replaces the shell's own process image, so a signal sent
  to the launcher's PID lands on the bundled node process natively, same
  PID, no relay. bin/rook.cjs's ~150 lines of SIGINT/SIGTERM/SIGHUP
  forwarding exist specifically because spawn() creates a child
  process that needs that relay — exec in a shell script doesn't have
  that problem to begin with.

Absolute paths baked into an installed script this way are ordinary,
relocatable Homebrew practice — any formula that writes a wrapper
referencing #{libexec} does the same; nothing new here.

Verification

Local brew install --build-bottle turned out to be broken in my
environment for reasons unrelated to this change — I confirmed this by
installing the already CI-verified, currently-live formula (pre-this-PR)
and getting the identical FormulaUnavailableError. Rather than fight that,
I pulled the real published @testmuai/rook and bundled-runtime npm
packages, reconstructed def install's exact pkg_dir layout by hand, and
ran the actual generated wrapper end to end with PATH=/usr/bin:/bin (no
node, no Homebrew, nothing):

$ env -i PATH="/usr/bin:/bin" ./rook --version
0.1.0
$ env -i PATH="/usr/bin:/bin" ./rook --help
Usage: rook [options] [command]
...

All 6 scripts/test-*.sh harnesses green, ruby -c, actionlint clean.

What it takes to actually ship

A bottle pours a pre-built keg — it never runs def install. Merging
this alone changes nothing for real users; def install's new code only
takes effect once build-bottles.yml rebuilds and republishes the 0.1.0
bottle. I'll dispatch that after merge and live-verify brew-smoke green
on both platforms before considering this done — the same live-verification
discipline that caught the #9 regression, this time before declaring
victory instead of after.

New test do assertion added as a structural guard on the actual
mechanism (checks the launcher's shebang, not just that rook --version
happens to work) — brew-smoke.yml's existing "node stays required" guard
is flipped back too, this time for the right reason.

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Agent-authored: Claude Sonnet 5, Claude Code CLI.