docs: Update CHANGELOG with sovereign infrastructure stack

Documents Strike 4A-Infra and Strike 4B completion:
- Gitea self-hosted Git (replaces GitHub)
- SeaweedFS object storage/CDN
- Gitea act_runner on asgard (16C/64GB)
- Companion agent build workflow
- Repository migration to sovereign infrastructure

Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>

CHANGELOG.md

@@ -33,6 +33,46 @@ All notable changes to this project will be documented in this file.
 **Infrastructure:**
 - Made `NatsBridge.jetstream` public for service consumer access
 
+### Added (Sovereign Infrastructure Stack)
+
+**Services Deployed:**
+- **Gitea** (git.corrosionmgmt.com) — Self-hosted Git with Actions support
+  - Container: `corrosion-gitea` on port 8090 (HTTP) and 8095 (SSH)
+  - SQLite database (self-contained, persistent)
+  - Replaces GitHub dependency for source control
+  - Gitea Actions enabled for CI/CD
+- **SeaweedFS** (cdn.corrosionmgmt.com) — S3-compatible object storage and CDN
+  - Container: `corrosion-cdn` with integrated Master/Volume/Filer/S3
+  - Filer UI at port 8091 (cdn.corrosionmgmt.com)
+  - Master UI at port 8093 (admin.cdn.corrosionmgmt.com)
+  - S3 API at port 8092 (internal access)
+  - Purpose: Map hosting, plugin packages, companion binaries, backups
+- **Gitea Act Runner** (asgard build server) — CI/CD execution environment
+  - Runs on Ryzen 9 7945HX (16C/32T, 64GB DDR5)
+  - Docker-based job execution
+  - Go 1.21+ and Rust toolchains available
+  - Connects to public Gitea instance remotely
+
+**CI/CD Workflows:**
+- `test-runner.yml` — Runner capability validation (hostname, resources, toolchains)
+- `build-companion.yml` — Production companion agent build pipeline:
+  - Triggers on version tags (v*.*.*)
+  - Cross-compiles for Linux AMD64 and Windows AMD64
+  - Generates SHA256 checksums
+  - Creates Gitea release with auto-generated installation instructions
+  - Uploads binaries and checksums as release assets
+
+**Documentation:**
+- `infra/docker-compose.yml` — Infrastructure stack definition
+- `infra/README.md` — Deployment guide and architecture overview
+- `infra/NPM-CONFIG.md` — Nginx Proxy Manager configuration
+- `infra/ASGARD-RUNNER.md` — Act runner setup guide
+
+**Repository Migration:**
+- Migrated from GitHub to self-hosted Gitea
+- Remote updated to `git@git.corrosionmgmt.com:vantzs/corrosion-admin-panel.git`
+- All future development on sovereign infrastructure
+
 ### Technical Details
 
 **Data Flow:**