Promote OpenCode from preview to core harness tier: live exporter, one live reply, then the flip #37

Origin

Split from #21 on 2026-09-23 under that issue's own definition of done ("or the OpenCode boxes moved to their own issue with this one closed on pi's two"). The original text and the 2026-09-20 re-scope stay on #21. Not an MVP blocker: the preview label is the honest state today and docs/agent-install.md already says so.

What exists today (from #21's 2026-09-20 re-scope, read from main)

CORE_HARNESSES = ("kiro", "codex", "claude", "pi"), PREVIEW_HARNESSES = ("opencode", "grok") (packages/studyloop/src/studyloop/harnesses.py). OpenCode's items 3 (session export) and 4 (live release check) fail live.

Boxes

• OpenCodeExporter reads ~/.local/share/opencode/opencode.db (SQLite session/message/part), pinned by a live-schema test, not another fixture; session-export --opencode-only lands source='opencode' rows from a real install.
• One live acceptance run with a completed model reply (a provider with quota — owner-gated), evidence bundle committed under the receipt directory.
• Then, and only then: CORE_HARNESSES gains "opencode", production=True, docs/spec/installer language in the same change, tests/test_docs_harness_tier_contract.py extended to pin it; council review of the evidence before the flip merges.

Definition of done

All three boxes ticked; full suite green; just lint; just typecheck. Do not flip the tier first and fix docs later.