chore: update Gateway SDK to alpha.12

[hopeatina]

OrgX OpenCode now resolves Gateway SDK alpha.12 from the immutable merged source commit 7acc1b37c8daab720c81515d16d2d9e9f0539a5d and advances the plugin package to 0.1.0-alpha.16.

Changes

• Pin @useorgx/orgx-gateway-sdk to the exact alpha.12 main merge.
• Record SDK version 0.1.0-alpha.12 and the exact source SHA in package-lock.json.
• Keep package.json, package-lock.json, plugin.manifest.json, and heartbeat expectations on OpenCode alpha.16.
• Preserve existing provider_id, observed_provider_id, HTTP recovery, and native context activation behavior.

Checks

• Focused attribution and context tests: 45 passed.
• npm run type-check: passed.
• npm test: 131 tests passed across 12 Vitest files and 8 Node tests.
• npm run build: passed.
• npm pack --dry-run --json: alpha.16, 83 files.

Release boundary

• Gateway SDK alpha.12 is not published to npm and has no GitHub release or tag; this PR uses its exact merged Git source contract.
• This PR does not tag or publish either package.