feature: {env:MY_VAR} support in the frontmatter of agent/command markdown files (resolves #5054, #5082)

[ariane-emory]

Resolves #5054. Resolves #5082.

feat: add {env:VAR} interpolation support to markdown frontmatter

• Add {env:VAR} interpolation to ConfigMarkdown.parse() for frontmatter fields only
• Enables dynamic model selection via environment variables in markdown agents, amongst other possibilities
• Gracefully handles missing environment variables with empty string fallback
• Add comprehensive test coverage for interpolation functionality
• Resolves GitHub issue #5054

Example usage:

description: "My agent" model: "{env:MODEL}" mode: primary

commit b9c1f100160d663e6372cf114d1d86d4c9575237 (upstream/dev, upstream/HEAD, origin/dev, origin/HEAD, dev)