feat(storage): add unified persistence layer for agent checkpoints and thread journals

[mikehostetler]

Summary

Implements the Jido Storage unified persistence layer based on the JIDO_STORAGE.md specification.

Changes

New Modules

• Jido.Storage - Unified storage behaviour with 6 callbacks for checkpoints and journals
• Jido.Storage.ETS - ETS adapter (ephemeral, for dev/test)
• Jido.Storage.File - File-based adapter (durable)
• Jido.Persist - Hibernate/thaw coordinator, invariant enforcer

Modified Modules

• Jido - Added storage: option to use Jido macro, hibernate/2 and thaw/3 functions
• Jido.Agent - Added checkpoint/2 and restore/2 callbacks with default implementations

Documentation

• Added comprehensive guides/storage.md guide (~800 lines)
• Added Storage modules to mix.exs docs configuration

Tests

• 87 new tests for storage functionality
• All 1479 tests pass
• 82.5% overall coverage (above 80% threshold)
• mix quality passes

Key Features

• Unified Jido.Storage behaviour - One adapter handles both checkpoints (overwrite) and journals (append-only)
• Thread pointer invariant - Checkpoint stores only %{id, rev} pointer, never full Thread
• Optimistic concurrency - expected_rev: option for conflict detection
• Agent callbacks - Optional checkpoint/2 and restore/2 for custom serialization

Usage

defmodule MyApp.Jido do
  use Jido,
    otp_app: :my_app,
    storage: {Jido.Storage.ETS, table: :my_storage}
end

# Hibernate an agent
:ok = MyApp.Jido.hibernate(agent)

# Thaw an agent
{:ok, agent} = MyApp.Jido.thaw(MyAgent, "user-123")