feat(installer): thin curl|bash shim over canonical CLI

[willackerly]

Summary

Adds setup-rebar.sh — a ~120-line one-line installer that clones rebar to ~/.rebar, runs the canonical bin/install for PATH/completion wiring, and exits pointing at rebar new / rebar adopt.

Deliberately a shim, not a parallel bootstrap codepath. Project creation stays in the Go CLI so this can't drift from rebar new, rebar adopt, bin/install, or ask init.

Background

Inspired by #1 (@ttschampel). That PR is the right signal — curl | bash is a real UX gap — but the implementation re-implemented rebar new in 462 lines of bash with several footguns (auto-Homebrew/bash5 + sudo, splatting bootstrap into \$PWD, blind git pull, never calling ask init or rebar init). Rather than merge-then-rewrite, landing the cleaner shim here on v3.0.0-alpha for review.

What it does

1. Clones (or updates) rebar to \$REBAR_DIR (default ~/.rebar)
2. Hands off to bin/install for PATH/completions/ASK_SERVER
3. Prints next steps: rebar new <name> or rebar adopt

That's it. No template copying, no auto-installing system deps, no project bootstrap — those live in the Go CLI where they belong.

Test Plan

• bash -n setup-rebar.sh — syntax clean
• ./setup-rebar.sh --help — usage prints correctly
• End-to-end: fresh dir → curl | bash from this branch → rebar new works
• --server HOST:PORT passthrough writes ASK_SERVER to RC
• Re-running over existing ~/.rebar checkout: dirty-check gates the fetch

Notes

• README points the curl URL at v3.0.0-alpha for now. When alpha lands on main, that pointer flips to main (or to a release tag) — that's the worked-example backport.
• Contracts: no contract impact (additive shell script + 4 README lines).

[willackerly]

@ttschampel — flagged this for you in #1. Would love your review since this is the cleaner version of what you originally proposed. Specifically curious whether anything felt obvious in your version that I shouldn't have dropped.