Update readme.txt: refresh description, tags, and SEO

[PatelUtkarsh]

Summary

Refresh the WordPress.org listing so Stream surfaces for activity log and audit log searches, and bring the plugin description up to date with recently shipped features.

Changes

• Display name: Stream – Activity Log & Audit Trail for WordPress (slug and installed Plugin Name: header unchanged)
• New short description built around activity/audit log terminology (137/150 chars)
• Tags: activity log, audit log, event log, user tracking, security (5-tag limit)
• Description intro rewritten; adds Abilities API / MCP Adapter support and reliable TTL auto-purge with orphaned-meta cleanup; adds open source note with GitHub link
• Adds missing Two Factor connector to the integrations list
• Changelog trimmed to the last 5 releases with user-facing notes inlined for 4.2.2–4.2.0 and anchor links to changelog.md (fixes two broken anchors); readme.txt now 9,156 bytes (~10 KB limit)
• readme.md tagline and stream.php header Description synced to the same terminology

Notes for release

• At the next release, demote the 4.2.0 inline changelog notes to a link to keep the file under the size limit.
• Post-release: verify Stream appears in WordPress.org search results for "activity log" and "log".

[bartoszgadomski]

Thank you @PatelUtkarsh! See my suggestions below: