docs: add [1.6.0] - TBD changelog section for marketplace availability

foundry-skills had no in-progress CHANGELOG section, so the marketplace availability gained since 1.5.0 was unrecorded. Add a [1.6.0] - TBD section (release.sh dates it at release) noting the plugin is now installable from the OpenAI/Codex, Cursor, and GitHub Copilot marketplaces (pending/unlisted at 1.5.0), with the OpenAI-required interface icon now in the bundle. Install-table link/command updates are already in the README. No release-tag link refs, matching existing convention.

Author: mraible

CHANGELOG.md

@@ -4,6 +4,12 @@ All notable changes to this project will be documented in this file.
 
 The format is based on [Keep a Changelog](https://keepachangelog.com/), and this project adheres to [Semantic Versioning](https://semver.org/).
 
+## [1.6.0] - TBD
+
+### Added
+
+- **Installable from the OpenAI/Codex, Cursor, and GitHub Copilot marketplaces.** Beyond the Anthropic marketplace, the plugin is now published to the OpenAI/Codex curated CLI marketplace (`codex plugin add crowdstrike-falcon-foundry@openai-api-curated`; ChatGPT-authenticated Codex installs via `/plugins`), the Cursor marketplace, and the GitHub Copilot (awesome-copilot) directory. The skills-only bundle now ships the square interface icon the OpenAI directory requires, and the README install table links each live listing.
+
 ## [1.5.0] - 2026-08-19
 
 ### Added