feat: Add CHANGELOG for v1.0.0 stable release

Document all features shipping in the initial stable release.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Author: zkoppert

CHANGELOG.md

@@ -0,0 +1,28 @@
+# Changelog
+
+All notable changes to this project will be documented in this file.
+
+## [1.0.0] - 2025-03-14
+
+### Initial stable release
+
+PR Conflict Detector is now considered stable and ready for production use.
+
+### Features
+
+- **Cross-repository conflict detection** - Scan an entire organization, a single repository, or a list of repositories for overlapping PR changes
+- **Line-level analysis** - Detects conflicts at the line range level, not just file overlap
+- **Same-author filtering** - Automatically excludes conflicts where both PRs are authored by the same person
+- **Conflict deduplication** - Tracks conflict history via GitHub Issues to only notify on new or changed conflicts
+- **Merge simulation verification** - Optionally verifies conflicts using GitHub's merge API (`VERIFY_CONFLICTS`)
+- **Draft PR support** - Optionally includes draft PRs in analysis (`INCLUDE_DRAFTS`)
+- **Author and team filtering** - Filter analysis to specific authors (`FILTER_AUTHORS`) or teams (`FILTER_TEAMS`)
+- **Multiple output formats**:
+  - Markdown report with GitHub Actions step summary
+  - JSON report for programmatic consumption
+  - GitHub Issues opened in affected repositories
+  - PR comments posted directly on conflicting PRs (`ENABLE_PR_COMMENTS`)
+  - Per-conflict Slack notifications with @mentions (`SLACK_WEBHOOK_URL`, `SLACK_CHANNEL`)
+- **Flexible authentication** - Supports GitHub App credentials, personal access tokens, and `GITHUB_TOKEN`
+- **GitHub Enterprise Server support** via `GH_ENTERPRISE_URL`
+- **Scalable** - Handles organizations with hundreds of open pull requests via pagination and rate limit awareness