feat: add experimental sf-multiframework extension

[bjwise96]

Summary

Adds an experimental sf-multiframework bundled extension for Salesforce Multi-Framework React UI Bundle work.

V1 is intentionally skill-first rather than tool-first:

• registers /sf-multiframework for status/checklist/Experience route reminders
• bundles skills/sf-multiframework/SKILL.md
• adds an Experience Cloud runbook for public routes, auth endpoints, cloned external profiles, Contact-linked external users, and sharing-vs-Apex-façade security choices
• routes implementation/verification to existing sf-pi surfaces (sf_apex, sf_soql, sf_browser, code_analyzer) instead of introducing a new native LLM tool prematurely

The extension is based on findings from an end-to-end Community Resilience Grants Multi-Framework demo with:

• an internal React CustomApplication
• an external React Experience UI Bundle
• public transparency route
• authenticated reviewer portal
• Apex REST APIs called via @salesforce/platform-sdk

Why

Multi-Framework work currently falls between generic LWC, Apex, metadata, and browser guidance. The main hard parts are template selection, current generated metadata shape, build artifact hygiene, and Experience Cloud access/security setup. A bundled sf-pi skill gives agents a first-class routing target for this work before they fall back to generic Salesforce skills.

Validation

• npm run generate-catalog:check
• npm run format:check
• npm run check -- --pretty false
• npm test -- extensions/sf-brain/tests/extension-context.test.ts

Notes

This PR intentionally does not add an sf_multiframework LLM tool. A future tool could inspect local UI Bundle shape, stale build artifacts, Experience metadata wiring, and package/import drift once the deterministic lifecycle surface is clearer.

[salesforce-cla]

Thanks for the contribution! Before we can merge this, we need @bjwise96 to sign the Salesforce Inc. Contributor License Agreement.