[JJERRYY]

Problem

sessionOf(req) previously looked only at the first role === "user" message and stopped after contents[0], ignoring x-magpie-session / x-magpie-subagent (and OpenCode's chat.headers sessionID).

In Codex (and other agents that prepend an AGENTS.md / <environment_context> message before the user's actual first prompt), req.contents[0].parts[0].text is identical across every thread (# AGENTS.md instructions...). As a result:

1. Every unrelated Codex conversation, background thread_title request, and concurrent subagent collided onto the exact same request.sessionId constant.
2. Interleaved requests across threads / subagents evicted each other's upstream KV cache beyond the shared ~32k preamble, causing frequent mid-session prompt cache drops.

Changes

• Read x-magpie-session and x-magpie-subagent in sessionOf(req, headers) (ignoring synthetic magpie-<24hex> fallback IDs, matching packages/cline), and forward OpenCode 1.x's sessionID via "chat.headers".
• When x-magpie-subagent is present (e.g. concurrent collab_spawn workers sharing a parent session ID) or when no host session header is provided, scan user parts past injected preambles (# AGENTS.md, <environment_context>, <permissions instructions>, <system-reminder>, Codex page snapshots, etc.) to reach the conversation's first real user prompt.
• Bump @magpie-community/opencode-antigravity-auth to 0.1.5, update README.md, and add unit tests in shape.test.mjs and requests.test.mjs.

Verification

• HOME=$(mktemp -d) bun test packages/antigravity: 88 pass, 0 fail
• bun scripts/check.mjs antigravity: exit 0