feat: add initContextual flag for automatic pageview tracking with page context

[MO-Thibault]

Summary

• Adds initContextual?: boolean to InitConfig — when true, the SDK automatically fires a pageview witness event with full page context during initialization (after passport init)
• Implies pageContext: true when no pageContext is explicitly configured, so callers only need one flag
• Follows the same non-blocking fire-and-forget pattern as initTargeting
• Documents both the flag and the manual witness(..., { includeContext: true }) approach in README

Motivation

Publishers integrating Optable for contextual audience assembly need to send page context on load. Previously this required custom integration code in each SDK bundle (e.g. the Playwire solution bundle). This flag makes it a first-class SDK feature with a single line of config.

Usage

const sdk = new OptableSDK({
  host: "dcn.customer.com",
  site: "my-site",
  initContextual: true,          // fire pageview + context on init
  // pageContext: { contentSelector: "article" }  // optional customization
});

Changes

• lib/config.ts — initContextual added to InitConfig, ResolvedConfig, and getConfig
• lib/sdk.ts — constructor defaults contextConfig to {} when initContextual is set without pageContext; initialize() fires the witness call
• README.md — new initContextual and pageContext entries in Optional Keys; new "Contextual Pageview Tracking" section under Witness API with automatic and manual usage examples

🤖 Generated with Claude Code

[jplaroche]

I believe URL is optional but we can keep it.
LGTM