feat: add typed client port with SDK adapter and test infrastructure

[chriswritescode-dev]

Summary

Adds a typed ForgeClient port interface that abstracts the opencode SDK v2,
an SDK adapter that normalises the {data, error} envelope with classified
errors, a FakeClient for hermetic testing, and refactors the existing
codebase to use the new port throughout.

Changes

Added

• src/client/port.ts — typed port interface with session, workspace, tui, and sync namespaces
• src/client/sdk-adapter.ts — SDK v2 adapter with error classification and combined factory
• test/client/sdk-adapter.test.ts — adapter test coverage
• test/helpers/fake-client.ts — fake client for hermetic test isolation
• test/helpers/fake-client.test.ts — fake client test coverage

Updated

• src/services/execution.ts, src/loop/runtime.ts, src/index.ts — core services migrated to ForgeClient
• src/hooks/* — all hooks updated to use the new port
• src/workspace/*, src/utils/* — utilities migrated to typed client
• src/tools/*, src/tui.tsx — tool and UI layer updated
• test/**/*.test.ts — all test files refactored to use FakeClient or the port
• vitest.config.ts — test config update

Removed

• Direct OpencodeClient v1 type usage across the codebase
• Inline SDK error handling — consolidated into sdk-adapter.ts

Breaking changes

None.