feat(harness): v2 host smoke test and config schema validation in CI (issue #27)

[diegohb]

C4 of spec #23.

What

• Verification harness (tests/v2-host.test.ts, tests/fixture-config-schema.test.ts + support classes) that loads the built package entry the way the v2 host does — pinned @opencode/plugin@2.0.23 module loading and Effect activation — and audits registrations against the real @opencode/schema: all ten agents decode against Agent.Info, exactly one permission evaluate hook, hook rewrites bundled external_directory asks end to end. Negative controls prove the audit fails on registration loss.
• Produced fixture configs (created default, splices into rich v2 configs, legacy-tolerated no-ops, README quick-start snippet) validated against the pinned Config.Info; v1-shaped configs rejected.
• Dedicated V2 verification harness CI job.

Key finding (facts record §14.6, settled negative)

Under Bun, Host.resolve's root-index fallback is unreachable: Bun.resolveSync throws ResolveMessage, which is not an instanceof Error, so the candidate-miss filter never catches and the core loader disables the plugin at the entry stage. The package had no exports map — consumers were silently registering nothing. exports["./server"] added; generated-package templates flagged in the record (§13 row 9) for the guidance tickets.

Facts write-back

docs/reference/opencode-v2-facts.md: §14.6 settled negative with mechanism; items 1,2,3,4,7,8 recorded as not reachable from a plugin-package harness; item 5 partially confirmed (pinned shapes); §7 amended — every plugin package must declare exports["./server"].

Ref #27