feat: WO-PS-001 initialize SDK repo

Initialize git.dev.alexdunmow.com/ninja/core with Go 1.26 module,
package directory structure, and README.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

README.md

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+# BlockNinja Plugin SDK
+
+Types, interfaces, and utilities for building BlockNinja plugins.
+
+## Package Structure
+
+| Package | Purpose |
+|---------|---------|
+| `plugin/` | PluginRegistration, ServiceDeps, RegisterFunc |
+| `blocks/` | BlockMeta, BlockFunc, BlockRegistry interface, BlockContext |
+| `blocks/builtin/` | Reusable block implementations (HTMLBlock) |
+| `templates/` | TemplateRegistry interface |
+| `templates/bn/` | Shared templ components (head, engagement, toolbar) |
+| `auth/` | Claims types, context extractors |
+| `content/` | Content access interface |
+| `settings/` | Settings access interface |
+| `gating/` | Access gating evaluation |
+| `crypto/` | Encrypt/Decrypt interface |
+| `render/` | Rich text rendering utilities |
+| `video/` | Video embed URL parsing |
+| `ai/` | AI tool registry interface and types |
+| `rbac/` | Role type definition |
+
+## Usage
+
+```go
+import "git.dev.alexdunmow.com/ninja/core/plugin"
+```

ai/doc.go

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+package ai

auth/doc.go

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+package auth

blocks/builtin/doc.go

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+package builtin

blocks/doc.go

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+package blocks

blocks/schemas/doc.go

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+package schemas

blocks/shared/doc.go

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+package shared

blocks/tags/doc.go

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+package tags

content/doc.go

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+package content

crypto/doc.go

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+package crypto

gating/doc.go

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+package gating

go.mod

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+module git.dev.alexdunmow.com/ninja/core
+
+go 1.26

plugin/doc.go

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+package plugin

rbac/doc.go

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+package rbac

render/doc.go

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+package render

settings/doc.go

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+package settings

templates/bn/doc.go

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+package bn

templates/doc.go

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+package templates

video/doc.go

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+package video