fix: bundle spec.md next to the CLI entry so the spec command works after build

[rmyndharis]

Summary

npx @google/design.md spec (and spec --rules / spec --format json without --rules-only) fails in an installed/published package with:

ERROR Failed to load spec.md.
  Bundled path: .../packages/cli/dist/spec.md
  Dev path:     .../docs/spec.md

Reproduction

cd packages/cli && bun run build
node dist/index.js spec        # throws "Failed to load spec.md"

Or from a packed tarball: bun pm pack (or npm pack), extract/install it elsewhere, and run node dist/index.js spec.

Root cause

bun build src/index.ts src/linter/index.ts --outdir dist bundles the CLI into dist/index.js, so at runtime getSpecContent() (Strategy 1 in spec-helpers.ts) resolves spec.md next to the executing module — i.e. dist/spec.md. The build script copies docs/spec.md only to dist/linter/, so dist/spec.md is absent. Strategy 2 (the dev fallback) then resolves ../../../../../docs/spec.md, which points outside an installed package, so the command throws.

spec --rules-only happened to keep working because that path doesn't read spec.md.

This mirrors spec-config.yaml, which the build already copies to both dist/ and dist/linter/ for exactly the same reason — spec.md was just missing the dist/ copy.

Fix

• Copy docs/spec.md to dist/ as well as dist/linter/ in the build script (one extra cp, consistent with the existing spec-config.yaml handling on the same line).
• Add npx design.md spec to the tarball smoke test so the bundled-spec.md resolution is exercised in CI. The existing Windows registry-smoke job only runs spec --rules-only, which skips spec loading, so this regression was not covered.

Testing

• bun test (packages/cli): 282 pass, 1 skip, 0 fail
• bun run lint (tsc --noEmit): clean
• bun run build then node dist/index.js spec, spec --rules, spec --format json: all exit 0 with full output
• Packed the tarball, extracted it, and ran node package/dist/index.js spec from the installed layout: exits 0 with the full spec (confirms dist/spec.md ships and resolves)

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