fix(opencode): support OpenCode 2 session_message schema and credential table

[arpankanwer]

Approved issue

Fixes #1124

TL;DR

OpenUsage failed on OpenCode 2 beta — "Couldn't read OpenCode's local database" and no Go meters — because the scanner still hit the v1 message table / auth.json only. This unions both message + session_message tables and falls back from auth.json to the SQLite credential table.

What was happening

OpenCode 2 (1.18.x beta, current stable) changed local storage:

• message (fields role, providerID, modelID, tokens.total) → session_message (columns type, seq, data with type='assistant', JSON model.providerID, model.id, tokens.input/output/reasoning/cache.* — no total)
• ~/.local/share/opencode/auth.json → SQLite credential table (integration_id='opencode-go', value JSON {"key":"sk-..."})

OpenUsage only queried FROM message with $.role / $.providerID / $.tokens.total and only read auth.json, so on a v2 DB every scan threw no such table: message or returned empty, and goAPIKey() returned nil.

What this changes

• OpenCodeUsageScanner: dataSQL/probeSQL now UNION ALL both tables with coalesced JSON paths
• OpenCodeAuthStore: try auth.json first, then fallback to credential table

Tests

swift build → Build complete
swift test --filter OpenCode → 40 passed

[github-actions]

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[arpankanwer]

@robinebers Thanks for assigning #1124! Could you please reopen #1186? Latest policy check is now passing, so it should stay open. I'll mark it Ready after.

[arpankanwer]

Superseded by #1243.

This PR could not be reopened: GitHub blocks it while the head branch has been force-pushed after close (state cannot be changed. The fix/opencode2-session-message-compat branch was force-pushed or recreated.). The replacement branch carries the same change, rebased onto current main, and #1243 links Fixes #1124 the same way.

Nothing to review here — please look at #1243 instead.