Parameterized markdown

[brillout]

Technically, how do we implement modes? [...] if the .md files are too similar, then maybe we really need a way to compose prompts.

Quoting:

• Open Loop: customizable prompts + flows, domain presets, marketplace #204 (comment)

My gut feeling says we will need a way to compose prompts from parameters. But, OK, let's first try the simpler frontmatter fan-out.

[suleimansh]

Tracking this as the post-#244 follow-up. Plan: ship the simple frontmatter fan-out in #244 first; do parameterized/composable prompts here only if the mode .md files come out too duplicative. Part of the Open Loop first slice (#204).