feat(codex): load shared memory on session start

[amit-shafnir]

What

Adds the Codex counterpart for NanoClaw's provider-agnostic memory system from #3012.

• Registers Codex's native SessionStart command hook for startup|clear|compact, excluding resume.
• Preserves unrelated Codex hooks and refreshes only NanoClaw's canonical memory entry.
• Explicitly disables Codex-native memory generation and use.
• Leaves Codex native model instructions intact.
• Stops injecting startup context on thread resume.
• Simplifies AGENTS.md composition now that shared memory is delivered through the native hook.

Why

Codex should consume the same agent-owned memory tree as Claude without overriding its native instructions or maintaining a second provider-specific memory store.

Testing

• Focused Codex provider tests: 15 passed
• Formatting and diff checks passed
• Successful fresh combined installation using core PR feat(memory): add provider-agnostic persistent memory #3012 and this providers payload

Dependency

Companion core PR: #3012. The core PR provides the shared memory files, hook contract, and installer integration consumed by this provider implementation.