refactor(protocol): extract server contracts

[kitlangton]

Summary

• add @opencode-ai/protocol as the canonical owner of the pure Effect HttpApi contract, endpoint groups, transport errors, and middleware service declarations
• move the remaining public DTO schemas needed by the contract into @opencode-ai/schema while preserving Core constructor and schema identity
• keep Server handlers, middleware layers, request parsing, database access, and Location service provisioning in @opencode-ai/server
• preserve existing @opencode-ai/server deep-import paths through direct compatibility re-exports

Architecture

The runtime dependency direction is now:

@opencode-ai/schema <- @opencode-ai/protocol <- @opencode-ai/server

Protocol has no runtime imports from Core. Location middleware retains one type-only Core dev dependency so Effect can carry the exact Location service union through handler construction; replacing that seam belongs to the next Protocol-owned service-algebra/embedded-facade slice rather than weakening middleware types here.

Compatibility

• preserves endpoint paths, operation identifiers, payloads, responses, errors, event unions, and middleware service identities
• preserves constructible Schema.Class APIs moved from Core, including FileSystem.FindInput
• preserves old Server API/group/error/middleware subpaths through direct re-exports
• replaces Node Buffer usage in SessionsCursor with Effect Base64URL encoding for browser-safe Protocol consumers
• generated OpenAPI and JavaScript SDK output have zero drift

Verification

• full workspace typecheck: 26/26 tasks
• focused HTTP/OpenAPI/SDK tests: 62 passed
• Schema constructor compatibility test passed
• Protocol browser-safe Session cursor round-trip test passed
• SDK regenerated with no diff in v1, v2, or committed OpenAPI output
• file-scoped Oxlint: 0 errors; remaining warnings are pre-existing Core warnings
• git diff --check passed
• three-agent simplify, package-boundary, and behavioral compatibility review completed; all actionable findings resolved