Add standalone installer (dexto.ai/install) + release artifacts

[rahulkarajgikar]

We want a low-friction install path for end users:

• curl -fsSL https://dexto.ai/install | bash installs a standalone dexto executable (no separate Bun/Node prerequisite).
• Keep existing npm-based install working in parallel.

Motivation

The repo is migrating to Bun for tooling/runtime, but requiring users to separately install Bun adds friction. A standalone executable + a curl install script avoids that and provides a predictable install/upgrade story.

Proposed approach

1. Produce standalone binaries for macOS (arm64/x64), Linux (x64/arm64; consider musl/baseline variants), Windows (x64).
   • Prefer compiling with Bun (Bun.build({ compile: ... })) to bundle the runtime.
2. Release workflow (GitHub Actions) builds artifacts and uploads them to GitHub Releases with consistent filenames.
3. Install endpoint https://dexto.ai/install serves a small shell script that:
   • detects OS/arch
   • downloads the correct artifact for latest (or a specified version) from GitHub Releases
   • installs into an XDG-friendly bin dir (e.g. $XDG_BIN_DIR, $HOME/bin, or $HOME/.dexto/bin)
   • optionally updates shell PATH
4. Optional hardening: checksums/signatures, retry logic, proxy support.

Open questions / decisions

• Where does the install script live/serve from? (dexto.ai infra vs GitHub Pages/proxying raw GitHub content)
• Do we also want an npm “wrapper” package that downloads/execs the correct platform binary?
• Version channels (stable vs dev/nightly) and how they map to releases/tags.

[linear]

TRU-353