Include tool catalog in system prompt

[bug-ops]

Parent: #239 (Phase A)

Summary

Format tool definitions from ToolRegistry into the system prompt so the LLM knows available tools.

Changes

• crates/zeph-core/src/context.rs: add tool definitions section to system prompt builder
• Format: tool id, description, parameters with types

Acceptance criteria

• System prompt includes tool catalog when tools are available
• Format is LLM-readable (structured text)
• Unit test for prompt generation with tools