Add Octopus Deploy Platform Hub knowledge base

New files:
- PLATFORMHUB.md: index for Platform Hub guidance
- platformhub/architecture.md: Git→PlatformHub→Projects model
- platformhub/ocl-syntax.md: complete OCL reference for templates
- platformhub/process-template-patterns.md: 5 proven patterns
- platformhub/gotchas.md: every error hit and how to fix it
- platformhub/api-reference.md: API vs UI capabilities
- best-practices/octopus-process-templates.md: consolidated best practices

Learned from building PlatformHub-Demo (30 microservices, 3 clouds).
Key discoveries: step templates are space-scoped (can't cross-reference),
worker_pool parameter is mandatory, publishing/sharing is UI-only.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

PLATFORMHUB.md

@@ -0,0 +1,11 @@
+# Octopus Deploy Platform Hub — Knowledge Base
+
+Practical guidance for working with Octopus Deploy's Platform Hub, learned from building the PlatformHub-Demo project (a mock 30-microservice FinTech marketplace). This is a thin index — detail lives in `platformhub/`.
+
+## Topics
+
+- [Architecture Overview](platformhub/architecture.md) — How Platform Hub fits together: Git repo, OCL files, spaces, publishing
+- [OCL Syntax Reference](platformhub/ocl-syntax.md) — Complete OCL format guide with examples for process templates
+- [Process Template Patterns](platformhub/process-template-patterns.md) — Proven patterns for different deployment types (Go, Python, Node.js, Payment, Database)
+- [Gotchas and Troubleshooting](platformhub/gotchas.md) — Every error we hit and how to fix it
+- [API Reference](platformhub/api-reference.md) — What can and cannot be done via API vs UI