fix(heroku): pin Node 22 and provide tree-sitter CLI so deploys succeed

[Kamehameya]

Problem

Heroku deploys for the playground app have been failing on every push — 43 of 43 builds failed. The build log only shows pnpm install → ELIFECYCLE Command failed with exit code 1, because the buildpack runs with NPM_CONFIG_LOGLEVEL=error and suppresses the detail.

Reproducing under Heroku-like conditions surfaced two independent blockers:

1. Heroku builds on Node 26, which can't compile the parser

engines.node was >=18.0.0. The Heroku Node buildpack prioritizes engines.node over the NODE_VERSION config var and resolves the open range to the newest release (Node 26.5.0) — the buildpack log even warns about the "wide range". The tree-sitter native module fails to compile against Node 26's V8 headers:

tree-sitter@0.25.0 ... node-gyp rebuild
error: variable template partial specialization of 'is_convertible_v' ...
gyp ERR! build error — `make` failed with exit code: 2
node -v v26.0.0
 ELIFECYCLE  Command failed with exit code 1.

This is why repeatedly setting NODE_VERSION on the app never took effect.

2. heroku-postbuild needs the tree-sitter CLI, which the buildpack lacks

Once Node is pinned and pnpm install succeeds (parser.c and prebuilds are committed), heroku-postbuild runs pnpm build, which calls tree-sitter generate:

> tree-sitter generate
sh: tree-sitter: command not found
 ELIFECYCLE  Command failed with exit code 1.

Fix

1. Pin engines.node to 22.x — matches mise.toml, CI, and the tested toolchain, and stops the buildpack from selecting Node 26.
2. Fetch the tree-sitter 0.25.10 CLI in heroku-postbuild into .heroku-bin/ and prepend it to PATH for the build, mirroring what .github/workflows/ci.yml already does. Added .heroku-bin/ to .gitignore.

Verification

Ran the exact heroku-postbuild script, unmodified, inside the real heroku/heroku:24-build image (emulated x86_64 to match dynos), with Node 22 and NODE_ENV=production, source copied in as a fresh slug:

• Base image already provides the C/C++ toolchain the parser needs: gcc 13.3, g++, make 4.3, python3.12, strip 2.42 (no Rust required — nothing on the build path invokes cargo).
• pnpm install --frozen-lockfile ✅
• heroku-postbuild build step ✅ — 16/16 turbo tasks green
• Output present: apps/ui/dist/index.html and apps/ui/dist/docs/index.html (what server.mjs serves) ✅

🤖 Generated with Claude Code

[salesforce-cla]

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