refactor(core): migrate accounts and load file agents

[thdxr]

Summary

• move the account service implementation, storage access, OAuth flow, and token refresh handling into @opencode-ai/core/account, then switch opencode consumers and tests to the core service
• load Markdown-backed agents from configured {agent,agents}/**/*.md directories in ConfigAgentPlugin using the same ConfigAgent.Info schema as JSON agent entries
• provide filesystem access to config-agent plugin bootstrapping and add gray-matter for frontmatter parsing

Validation

• cd packages/core && bunx prettier --check "src/account.ts" "src/account/sql.ts" "src/config/plugin/agent.ts" "src/plugin/boot.ts" "test/account.test.ts" "test/config/agent.test.ts" "package.json"
• cd packages/core && bun typecheck
• cd packages/core && bun test test/account.test.ts test/config/agent.test.ts
• cd packages/opencode && bunx prettier --check "src/cli/cmd/account.ts" "src/config/config.ts" "src/effect/app-runtime.ts" "src/server/routes/instance/httpapi/groups/experimental.ts" "src/server/routes/instance/httpapi/handlers/experimental.ts" "src/server/routes/instance/httpapi/server.ts" "src/share/share-next.ts" "test/cli/error.test.ts" "test/config/config.test.ts" "test/fake/account.ts" "test/share/share-next.test.ts"
• cd packages/opencode && bun typecheck
• cd packages/opencode && bun test test/cli/error.test.ts test/config/config.test.ts test/share/share-next.test.ts
• bun turbo typecheck (pre-push hook)

Known Failure

• cd packages/core && bun test test/location-layer.test.ts currently fails during database setup with SQLiteError: table 'permission' already exists, before reaching the changed plugin behavior.

[greptile-apps]

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