feat(plugin): expose session history reads

[opencode-agent]

Summary

• expose paginated persisted session enumeration through ctx.session.list
• add ctx.session.children for direct child traversal and ctx.session.messages for retained message history
• support both Effect and Promise plugin entrypoints with the same bounded cursor behavior as the server APIs
• document the new plugin capabilities

Validation

• bun typecheck (packages/client)
• bun typecheck (packages/plugin)
• bun typecheck (packages/core)
• bun test test/plugin.test.ts test/plugin/promise.test.ts (packages/core)
• Prettier check for changed files

Requested by: @rekram1-node (Aiden via Slack)