Centralize non-sensitive config into per-app modules

[Aliiiu]

Problem

Non-sensitive configuration is scattered as inline literals and direct process.env reads across several apps in the monorepo, with no single source of truth — changing a value means hunting for every occurrence:

• docs — PostHog host strings are hardcoded in two places that must stay in sync: the client init (instrumentation-client.ts) and the build-time /ingest rewrites (next.config.mjs).
• shade-agent — NEXT_PUBLIC_contractId is read directly via process.env in two route files, bypassing the app's existing validated env module.
• examples — the default model name is duplicated as an inline literal across 13 agent files (two distinct defaults).

Proposal

Centralize each app's non-sensitive config into one source-of-truth module, with zero behavior change. Rule: read from process.env only where the env var is a documented per-deploy / user-override value (e.g. LLM_MODEL, contractId, the PostHog key); otherwise hold a plain constant (fixed hosts).

• docs — apps/docs/config/posthog.mjs (plain ESM so both the client init and next.config.mjs can import it).
• shade-agent — add NEXT_PUBLIC_contractId to the existing Zod env.ts (optional, to preserve graceful runtime handling).
• examples — apps/examples/src/config.ts exporting DEFAULT_MODEL / DEFAULT_MODEL_STABLE.

Out of scope

Sensitive keys/tokens stay as direct process.env reads; model names in JSDoc/comments are left as-is.