feat(instance): add instance isolation support for multi-tenant deployments

[mikehostetler]

Summary

Adds instance isolation support for multi-tenant deployments where each tenant can have completely isolated signal infrastructure.

Changes

New modules:

• Jido.Signal.Names - Resolves process names based on jido: option
• Jido.Signal.Instance - Child spec for starting isolated supervisor trees

Updated modules:

• Jido.Signal.Util - via_tuple/2 and whereis/2 support jido: option
• Jido.Signal.Bus - Stores jido option in state
• Jido.Signal.Bus.State - Added jido field
• Jido.Signal.Ext.Registry - Added child_spec/1 for instance naming

Usage

# Start isolated instance
{:ok, _} = Jido.Signal.Instance.start_link(name: MyApp.Jido)

# Bus uses instance-scoped registry
{:ok, _} = Bus.start_link(name: :tenant_bus, jido: MyApp.Jido)

# Multiple instances are completely isolated
{:ok, _} = Instance.start_link(name: TenantA.Jido)
{:ok, _} = Instance.start_link(name: TenantB.Jido)

# Same bus name, different instances = different processes
{:ok, bus_a} = Bus.start_link(name: :events, jido: TenantA.Jido)
{:ok, bus_b} = Bus.start_link(name: :events, jido: TenantB.Jido)

Documentation

• Updated README, AGENTS.md, usage-rules.md
• Added Instance Isolation section to guides/event-bus.md
• Added testing patterns to guides/advanced.md

Tests

• 14 new tests for Names, Instance, and Bus isolation