docs(readme): update Quick Start to reflect collection-based extension and installer options

[katriendg]

Summary

Update the root README.md Quick Start section to reflect collection-based installation options for the VS Code extension and the hve-core-installer agent.

Problem

The current README Quick Start presents three install paths:

1. VS Code Extension — marketplace install only, no mention of collection-based filtering
2. Copilot CLI Plugin — already shows collection-based CLI install (correct)
3. Automated Agent — hve-core-installer, no mention of its collection selection capability

The extension now supports collection-based packaging (via Package-Extension.ps1 -Collection), allowing users to install a curated subset of artifacts. The hve-core-installer agent already has a "Install by collection" sub-flow (option 2 in its Phase 3 selection menu). Neither capability is surfaced in the README.

Scope

Extension section updates

• Mention that collection-based extension variants are available (e.g., rpi, ado, prompt-engineering) alongside the full hve-core extension
• Link to the extension install guide for details on collection variants
• Clarify the default marketplace install gives the full hve-core-all set

Installer section updates

• Add a note that the installer supports collection-based install (option 2: "Install by collection")
• Mention that users can select specific domain collections instead of the full suite
• Briefly describe the collection selection flow

Project Structure section

• Add collections/ directory to the project structure tree (currently absent)

What's Included table

• Consider adding a row for Collections showing the domain count

Acceptance Criteria

• README Quick Start extension section mentions collection-based variants
• README Quick Start installer section mentions collection selection capability
• Project Structure tree includes collections/ directory
• Links to detailed install guides remain accurate
• No information about unshipped or unreleased features (only document what is available)

[katriendg]

Dependent on #519 and associated PR merged first.