#14026: gateway: expose fork/upstream identity metadata with override

Upstreaming gateway identity metadata from DeanoC/openclaw#15 in a clean branch based on upstream/main. Adds a lightweight identity signal in WS hello metadata so clients can detect fork vs upstream and support compatibility behavior. Included in this upstream slice: - `hello-ok.server.identity` with `kind`, `mode`, `source` - identity mode override via config/env - schema/type wiring - focused tests for resolver + hello payload - docs updates (protocol/config/index) Related fork PR: https://github.com/DeanoC/openclaw/pull/15 Earlier upstream attempt (superseded by this clean branch): https://github.com/openclaw/openclaw/pull/14025 <!-- greptile_comment --> <h2>Greptile Overview</h2> <h3>Greptile Summary</h3> Adds lightweight identity metadata to gateway `hello-ok` handshake, enabling clients to distinguish fork vs upstream instances. The implementation includes: - New `gateway.identity.mode` config option (`auto|upstream|fork`) with env override via `OPENCLAW_GATEWAY_IDENTITY_MODE` - Identity resolver that respects precedence: env > config > default - Schema wiring for `hello-ok.server.identity` with `kind`, `mode`, and `source` fields - Comprehensive unit tests for the resolver logic - E2E tests validating the identity appears correctly in handshake responses - Documentation updates across protocol, config, and gateway index The change is well-isolated, follows existing patterns for config/env resolution, and provides the compatibility testing capability described in the PR description. <h3>Confidence Score: 5/5</h3> - This PR is safe to merge with minimal risk - The implementation is clean, well-tested, and narrowly scoped. All new code follows established patterns (config resolution with env override), includes comprehensive test coverage (unit + E2E), and doesn't modify any critical paths. The identity metadata is purely informational and consumed by clients, not used for authorization or control flow. - No files require special attention <!-- greptile_other_comments_section --> <!-- /greptile_comment -->