Embed core/components in the xmd binary with a default component search path

[taras]

Motivation

The PR-review and repo-analysis workflows run on the released xmd binary but
still pass --component-dir core/components, pointing at the checked-out repo
(see .github/workflows/review.yml and .github/workflows/repo-analysis.yml).
core/components are framework provider components (AnthropicProvider.md,
Instruction.md, Sample.md, …) that should ship inside the binary, so a
consumer running the binary doesn't need the repo's core/components on disk.

Goal

Bundle core/components into the xmd binary and add a default component
search path so --component-dir core/components is no longer required.

Notes / investigation

Unlike @executablemd/code-review-agent (a module resolved by import, bundled
via deno compile --include), core/components are .md files read from disk
by path at runtime. A compiled binary can't read them by their repo-relative
path, so this needs runtime component-resolution work:

• Embed the .md components at compile time (--include core/components) and
  confirm they're readable at runtime from the compiled binary.
• Add a default embedded component directory to the CLI's component resolution
  so bundled components are found without --component-dir.

Refs

Deferred from PR #82 (review runs on the released binary). Once done, drop the
--component-dir core/components flag from both workflows.