fix: agent_sdk.zig — deduplicate doc comments and remove unused import (#427)

[yxlyx]

Summary

• Removed 3x duplicated doc comments on AgentOptions, tryClaudeAgent, and streamClaudeOutput
• Removed unused const mj = @import("mcp").json import (line 16)
• Net: 12 lines deleted, zero behavior change

Test plan

• zig ast-check src/agent_sdk.zig passes
• grep confirms no mj. usage in the file
• Each public item now has exactly one doc comment block

Fixes #427

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