Preserve legacy fallback for SDK backend

[Nomadcxx]

Summary

• Keep existing binary-backed workflows working in automatic mode, while using the SDK path only when explicitly selected or as a real-key fallback.
• Split legacy auth from SDK API-key auth, preserve OpenCode-owned MCP execution, and fix packaged runner path resolution.
• Make the SDK dependency optional, restore source install symlink behavior, and keep model discovery on the legacy path when available.

Test plan

• npm ci --omit=optional && npm ci
• npm run build
• npm run test:ci:unit (323 pass)
• npm run test:ci:integration (30 pass, 1 existing skip)
• SDK runner no-key smoke: expected CURSOR_API_KEY not set
• npm pack --dry-run --json includes dist/plugin-entry.js and scripts/sdk-runner.mjs

Note: a live SDK request was not run because no real CURSOR_API_KEY was available locally.

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