[CONTRIBUTION] Manifest.build preset

[mhenke]

Findings: A community member shared a working preset that routes oh-my-opencode-slim agents through the self-hosted Manifest router (tier-based: simple, standard, complex, reasoning), including the preset JSON and the OpenCode provider config. Offered as a contribution.

What was said:

• @FirebirdRender: built and shared a Manifest.build preset that maps each agent to a Manifest tier via headers, with failover across sources; included both the plugin preset and the opencode.json provider snippet.

Open questions:

1. Should curated community presets like this live in the repo (for example a presets folder) or stay in discussions?
2. Does the tier-based routing approach generalize well across other self-hosted routers?
3. Would a short "community presets" doc page be more useful than scattered preset shares?

Original issue: #456