feat: Step 6 - Write-through cache API

Implement the core write-through cache pattern for node creation.
This is the architectural foundation of the application.

Changes:
- Add @google/generative-ai dependency for embeddings
- Create lib/db.ts: SurrealDB connection helper with JWT auth
- Create lib/ai.ts: AI embedding generation using text-embedding-004
- Create app/api/nodes/route.ts: POST endpoint implementing write-through cache

Write-through cache flow:
1. Authenticate user via SurrealDB JWT
2. Publish node to ATproto PDS (source of truth)
3. Generate 768-dimensional embedding via Google AI
4. Cache node + embedding + links in SurrealDB

Updated schema to use 768-dimensional embeddings (text-embedding-004)
instead of 1536 dimensions.

Security:
- Row-level permissions enforced via SurrealDB JWT
- All secrets server-side only
- ATproto OAuth tokens from secure cookies

🤖 Generated with [Claude Code](https://claude.com/claude-code)

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

AGENTS.md

@@ -3,6 +3,8 @@
 This document outlines the standards and practices for the development of the
 Ponderants application. The AI agent must adhere to these guidelines strictly.
 
+**Git Commits**: For this project, you can skip PGP-signed commits and run git commands directly. You have permission to execute git add, git commit, and git push commands yourself.
+
 You are an expert-level, full-stack AI coding agent. Your task is to implement
 the "Ponderants" application. Product Vision: Ponderants is an AI-powered
 thought partner that interviews a user to capture, structure, and visualize
@@ -61,6 +63,11 @@ on clean, expert-level implementation adhering to the specified architecture.
   access their own data.
 - **Secrets Management:** Store all API keys and secrets in environment
   variables.
+- **Environment Variables:** NEVER set default values in code for configuration
+  variables. All configuration must come from .env file. Code should throw
+  an error with a clear message if required environment variables are missing.
+  This ensures configuration issues are caught immediately rather than silently
+  falling back to potentially incorrect defaults.
 - **Input Validation:** Validate all inputs on the server side (e.g., using
   Zod).
 
@@ -231,3 +238,8 @@ You, the AI Agent, MUST adhere to the following principles at all times. These a
 ### **Aesthetic Goal**
 
 The user has demanded a "stunningly-beautiful" application. You will achieve this not with complex, custom CSS, but with the thoughtful and elegant use of Mantine's layout components (\<Stack\>, \<Group\>, \<Paper\>, \<AppShell\>), a sophisticated grayscale theme.ts, and fluid interactions. The 3D visualization, powered by React Three Fiber, must be smooth, interactive, and beautiful.
+
+### MCP Servers
+
+You have access to SurrealDB and playwright MCP servers -- use them readily to
+help debug any issues you may encounter!