test(opencode): complete Phase 4 testing and polish

[edheltzel]

Summary

• Align the OpenCode adapter with the current 1.18.4 event and JSON export contract.
• Add an isolated OpenCode runtime e2e covering export, markdown drop, retry, concurrent WAL writers, searchable retrieval, and installer rollback.
• Preserve JSONC and unrelated MCP entries during OpenCode uninstall; document the operational contract.

Scope

• Testing and polish only; no semantic Dead semantic tier is reported but never escalated — silent keyword-only degradation destroys recall (field report for #226 item 3) #240/recall repair --execute leaves the vec index empty — semantic search silently degrades to bruteforce, and doctor reports PASS #241/Harden vec index: zero-vector guard, index-format version stamp, doctor/stats vec observability #226 work, release/version bump, or local installation reconciliation.
• Runtime, OpenCode config, Recall DB, and host files in the e2e are disposable and isolated.
• Related issues: 10 lifecycle tests fail in any git worktree — the environment AGENTS.md mandates for workers has a permanently red baseline #243 and chore: fresh /ce-worktree worktrees miss node_modules → ~10 MCP-config tests fail spuriously #165. Neither is closed by this PR.

Validation

• bun run test:e2e:opencode passed, with OpenCode 1.18.4 provisioned in disposable HOME/XDG roots.
• Codex, Claude Code, and Pi package e2e checks passed.
• Focused Phase 4 suite: 90 passed.
• bun run build, bun run lint, check:version, shell syntax, and diff checks passed.
• Full suite: 1,266 passed and one unchanged baseline Claude JSONL archive-scrub test failed in isolation because its expected LoA transcript was absent; this is outside Phase 4 scope and is documented in the handoff.