B: Build the xmd binary via deno compile

[taras]

Part of the executable.md v1 public release milestone.

Build a standalone xmd binary with deno compile. Validated feasible: runs eval + no-eval docs offline; slim flags cut size 274 MB → 86 MB.

Tasks

• Add a build task to deno.json (root config = full import map; no separate build config file):
  deno compile --node-modules-dir=none --exclude-unused-npm --exclude .opencode --exclude .internal --allow-all --output dist/xmd cli/src/cli.ts
• Confirm slim binary runs smoke-test/README.md and a StubProvider doc offline (exit 0).
• Parameterize for the cross-compile matrix (used by the release workflow): aarch64-apple-darwin, x86_64-apple-darwin, x86_64-unknown-linux-gnu, aarch64-unknown-linux-gnu, x86_64-pc-windows-msvc.exe; artifacts xmd-<triple>.

Notes

• Default 274 MB comes from nodeModulesDir: "auto" embedding the whole local node_modules (dev-tool platform binaries). Managed-npm mode fixes it; residual ~65 MB is the Deno runtime.
• --allow-all for v1 (see permission-proxy follow-up issue).

[taras]

Build task landed in #71 (deno task build, slim 86 MB binary, verified runs docs offline). Cross-compile matrix is wired in #72.