Add `xmd run -e "<inline document>"` to run inline content without a file

[taras]

Follow-up from #75.

The website hero wants a self-contained first command that doesn't reference a file the visitor doesn't have. The natural fit is an inline form of run:

xmd run -e "<inline EMA document>"

e.g. the motivating demo:

xmd run -e "<ClaudeCode>Hello World</ClaudeCode>"

Scope

• -e / --eval flag on run (cli/src/cli.ts): make docPath optional and accept an inline content string instead; run it through the same pipeline (e.g. write to a temp doc or add a content path to runDocument in core/src/run-document.ts, which today only takes docPath).
• For the <ClaudeCode> demo to actually work out-of-the-box it also needs:
  • a shipped/embeddable ClaudeCode component (a provider that shells out to the claude CLI) — relates to embedding built-in components (Embed built-in components in the xmd binary #67), since inline docs have no component-dir,
  • the claude CLI present on the user's machine.
• Pick a simpler inline example if the ClaudeCode path isn't ready (e.g. an inline bash exec block works with zero external deps).

Owner (Taras) plans to implement this separately.