Prepare Skillwalker for Homebrew installs

[mxriverlynn]

Prepares Skillwalker to be installed with Homebrew. The formula and the testdouble/homebrew-tap repo come later. This PR makes the in-repo changes a formula needs, following the investigation.

What changes

• --version reports a real version. scripts/build.ts passes packages/cli/package.json's version into the compiled binary. Running from source reports dev. Before this change, the binary printed unknown.
• Compiled binaries are re-signed on macOS. bun build --compile leaves a signature that codesign --verify rejects, and recent macOS releases kill such binaries on launch. The build now strips the stale signature, signs ad hoc, and verifies. It fails if any step fails.
• SKILLWALKER_SCRIPTS_DIR points the CLI at the sandbox scripts. sandbox create mounts the scripts folder into the sandbox. A Homebrew install's own folder changes with every version, so without this setting each brew upgrade would force sandbox update and a new Claude login. If the folder is missing a script, startup fails with an error that names the variable.
• Error messages say skillwalker, not ./build/skillwalker. Five sandbox retry hints pointed at a path that won't exist after a Homebrew install.
• Release workflow. Pushing a v* tag fails unless the tag matches the CLI package version. It then builds and smoke tests on arm64 and x86_64 macOS, and attaches skillwalker-<version>-darwin-<arch>.tar.gz plus .sha256 files to a draft GitHub Release.
• macOS CI job. Smoke tests the compiled binaries on macos-15, where the signature check runs.

One change from the investigation plan

The plan read the version from a SKILLWALKER_VERSION environment variable, falling back to package.json. This PR reads it only from packages/cli/package.json, and the release workflow checks the tag against it. That leaves one source for the version.

Tests

• Unit tests:
  • the dev version fallback
  • resolveSandboxScripts with the variable unset, set, relative, and missing a script
  • the five sandbox error messages
• Smoke tests (make build && bun run test:smoke):
  • --version
  • running through a bin symlink into a libexec copy from another directory
  • SKILLWALKER_SCRIPTS_DIR, both valid and empty
  • codesign --verify on macOS

The symlink and scripts-folder smoke tests passed on their first run, because the behavior already existed. I broke the layout on purpose (left duckdb.node out of the copy) to confirm the symlink test goes red.

Local results on macOS 26.5.1, arm64 (first run on Bun 1.3.11, re-run on 1.4.2):

• make test: 83 files, 1080 tests passed
• bun run test:smoke: 8 passed
• bun run typecheck and bun run format:check: pass
• bun run lint: 263 warnings, the same count as main

Before merging or tagging

• Confirm macos-15-intel is a valid GitHub-hosted runner label.
• Confirm oven-sh/setup-bun@v2 accepts bun-version-file: package.json (it should read the packageManager pin, Bun 1.4.2).
• Re-ran locally on the pinned Bun 1.4.2: make test 1080 passed, bun run test:smoke 8 passed, typecheck passed. On 1.4.2, a plain bun build --compile of the CLI already passes codesign --verify, so the re-sign step now only protects against a Bun downgrade or regression. The smoke test would catch either of those on its own.
• Neither workflow change has run on GitHub yet. The macOS CI job will run on this PR.

Not in this PR

• The Homebrew formula and the testdouble/homebrew-tap repo.
• Automatic formula updates on release (needs the tap and a HOMEBREW_TAP_TOKEN secret).
• README install instructions for brew install, which wait until the tap exists.