Refactor tool config: replace global state with proper dependency injection

[tcdent]

Problem

Tools currently access config through global state (e.g., OnceLock<AgentContext> in spawn_agent) because the app doesn't create tools directly - they're instantiated via Tool::execute().

This pattern is used by:

• spawn_agent.rs: AGENT_CONTEXT: OnceLock<AgentContext>
• fetch_html (PR Add Chrome profile support to fetch_html tool #45): loads config per-call, should use same pattern for consistency

Current workarounds

1. Global OnceLock initialized at app startup
2. Loading config inside the tool handler (inefficient)

Desired solution

Pass config/context through the effect handler system so tools don't need global state. Options:

• Add a Context parameter to EffectHandler::call()
• Use a trait object for tool context in ToolExecutor
• Dependency injection at tool registration time

Affected code

• src/tools/impls/spawn_agent.rs - AGENT_CONTEXT
• src/tools/handlers.rs - effect handlers
• src/tools/exec.rs - ToolExecutor