test: unit tests for agent SDK transport, session store, and hook log

[justrach]

Summary

Add unit tests for the new Agent SDK integration layer.

Test targets

src/agent_sdk.zig

• test: agent_sdk falls back to codex_appserver when SDK not found
• test: agent_sdk buildAgentArgs serialises allowed_tools array correctly
• test: agent_sdk buildAgentArgs serialises mcp_servers map correctly

src/sessions.zig

• test: session store evicts LRU when capacity exceeded
• test: session store touch updates last_used timestamp
• test: session store list returns all active sessions

src/tools.zig (hook log)

• test: hook log serialises to valid JSON array
• test: hook log block action prevents tool call entry

Files

• src/agent_sdk.zig: test block
• src/sessions.zig: test block
• src/tools.zig: test block

[justrach]

Blocked by: #179.

[justrach]

Blocked by: #178.