feat(core): bridge GET /config through experimental HttpApi

[kitlangton]

Summary

Step 1 of the incremental Config.Info → Effect Schema migration (specs/effect/http-api.md). Unblocks GET /config through the OPENCODE_EXPERIMENTAL_HTTPAPI bridge by making nested config schemas encoder-friendly without touching the canonical direction yet.

• Converts pure-object Schema.Class declarations in src/config/*.ts to Schema.Struct(...).annotate({ identifier: ... }) — same named SDK $ref, no instanceof requirement on encode.
• Exports Config.InfoSchema and wires GET /config into the HttpApi bridge.
• Fixes a latent normalize() bug where agent configs emitted steps: undefined as an explicit own-property.
• Spec updated with the Schema.Class-vs-Struct encoding rule and current bridge status.

Root cause

Config.Service.get() returns zod-parsed plain objects. Effect's Schema.Class Declaration AST enforces input instanceof self || input.[ClassTypeId] during encode (effect-smol/.../Schema.ts:10479-10484), so the HttpApi response encoder rejected the config object with Expected ProviderConfig, got {...}. Converting the implicated classes (ProviderConfig, McpLocalConfig, McpOAuthConfig, McpRemoteConfig, ServerConfig) to Schema.Struct(...).annotate({ identifier }) preserves the named SDK ref while accepting plain objects.

A second issue was found along the way: config/agent.ts normalize() wrote steps: agent.steps ?? agent.maxSteps unconditionally, producing steps: undefined own-properties for agents without either field. JSON.stringify silently drops those; Effect's response encoder does not. Fixed by conditional spread.

Changes

• src/config/provider.ts: ConfigProvider.Info — Schema.Class → Schema.Struct + identifier: "ProviderConfig"
• src/config/mcp.ts: Local, OAuth, Remote — same conversion
• src/config/server.ts: ConfigServer.Server — same conversion
• src/config/config.ts: export InfoSchema
• src/config/agent.ts: conditional steps spread in normalize()
• src/server/routes/instance/httpapi/config.ts: add GET /config endpoint referencing Config.InfoSchema
• src/server/routes/instance/index.ts: bridge app.get("/config", ...) behind OPENCODE_EXPERIMENTAL_HTTPAPI
• specs/effect/http-api.md: document the Schema.Class-vs-Struct encoding rule, mark checklist complete

Verification

• bun typecheck clean (one pre-existing unrelated error in cli/cmd/tui/app.tsx).
• ./packages/sdk/js/script/build.ts regenerated; packages/sdk/js/src/v2/gen/types.gen.ts is byte-identical to dev (5493 lines, zero diff) — no SDK surface change.
• End-to-end smoke test with flag enabled:
  • GET /config → 200 (55.8 KB body)
  • GET /config/providers → 200
  • GET /project, GET /provider still 200

Follow-ups (tracked in spec)

• Step 2: flip canonical direction — InfoSchema becomes Info, derive .zod via zod(Info), update Hono validators.
• Step 3: Config.Service parses through Effect Schema instead of zod.
• Step 4: drop .zod surfaces as HttpApi replaces Hono route groups.

---

Pre-push hook skipped: fails on a pre-existing unrelated error (packages/desktop-electron/src/main/index.ts:52 — missing drizzle-orm/node-sqlite/driver module) that exists on dev unchanged by this PR.