Add package documentation to all packages

[fogfish]

Problem: Some packages lack package-level godoc comments.

Required Changes:

1. Add doc.go or package comment to:
   • agent/ package
   • memory/ package
   • reasoner/ package
   • codec/ package
   • prompt/jsonify/ package

Template (agent/doc.go):

// Package agent provides implementations of the three core agent types:
// Prompter (stateless), Manifold (tool-enabled), and Automata (full agent).
//
// Usage:
//
//   // Simple stateless agent
//   prompter := agent.NewPrompter(llm, encoderFunc)
//   result, _ := prompter.Prompt(ctx, input)
//
//   // Tool-enabled agent
//   manifold := agent.NewManifold(llm, encoder, decoder, registry)
//   result, _ := manifold.Prompt(ctx, input)
//
//   // Full autonomous agent
//   automata := agent.NewAutomata(llm, memory, encoder, decoder, reasoner)
//   result, _ := automata.Prompt(ctx, input)
//
// See package examples for detailed usage patterns.
package agent

Estimated Effort: 1.5 hours
Skills Required: Technical writing, godoc