Incorporate art-dag-mono repo into artdag/ subfolder

Merges full history from art-dag/mono.git into the monorepo
under the artdag/ directory. Contains: core (DAG engine),
l1 (Celery rendering server), l2 (ActivityPub registry),
common (shared templates/middleware), client (CLI), test (e2e).

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

git-subtree-dir: artdag
git-subtree-mainline: 1a179de547
git-subtree-split: 4c2e716558

artdag/core/effects/identity/README.md

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+# Identity Effect
+
+The identity effect returns its input unchanged. It serves as the foundational primitive in the effects registry.
+
+## Purpose
+
+- **Testing**: Verify the effects pipeline is working correctly
+- **No-op placeholder**: Use when an effect slot requires a value but no transformation is needed
+- **Composition base**: The neutral element for effect composition
+
+## Signature
+
+```
+identity(input) → input
+```
+
+## Properties
+
+- **Idempotent**: `identity(identity(x)) = identity(x)`
+- **Neutral**: For any effect `f`, `identity ∘ f = f ∘ identity = f`
+
+## Implementation
+
+```python
+def identity(input):
+    return input
+```
+
+## Content Hash
+
+The identity effect is content-addressed by its behavior: given any input, the output hash equals the input hash.
+
+## Owner
+
+Registered by `@giles@artdag.rose-ash.com`

artdag/core/effects/identity/requirements.txt

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+# Identity effect has no dependencies
+# It's a pure function: identity(x) = x