refactor: flatten macro runtime and remove base engines

[chhoumann]

Summary

• Extracts macro execution context and command running into flat, explicit runtime components.
• Routes inline JavaScript and macro/template token evaluation through compatibility-preserving evaluator seams.
• Deletes obsolete inheritance-based engine files and refreshes architecture notes after the cutover.

Stack

1. engine-flat/01-vault-frontmatter-services
2. engine-flat/02-formatter-template-services
3. engine-flat/03-template-choice
4. engine-flat/04-capture-choice
5. This PR

Validation

• bun run lint
• bun run test
• bun run build
• Obsidian dev vault plugin reload and e2e validation

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View logs

[chhoumann]

Closing this stack after deciding not to continue with this refactor direction.