feat: add runtime context runnable API

[mrcrgl]

Summary

• add RunnableWithContext as a convenience API for runnables that should not manage RuntimeGuard directly
• add RuntimeContext with tick(...) and handle() accessors
• add RuntimeContextRunnable and with_runtime_context(...) adapter so context-based runnables work with ProcessManager
• add integration coverage proving reload/shutdown handling works with the context-based API
• add new runnable example: examples/runtime_context.rs
• include the new example in CI execution and update docs/changelog

Validation

• cargo fmt --all -- --check
• cargo clippy --all-features -- -D warnings
• cargo test --all
• cargo test --no-default-features
• timeout 30s cargo run --example simple
• timeout 45s cargo run --example dynamic_add
• timeout 30s cargo run --example restart_supervisor
• timeout 30s cargo run --example runtime_context

Closes #21