ticket: close workspace backend config schema

.yoi/tickets/00001KWF2210A/resolution.md

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+Implemented Workspace Backend config file schema and resolver.
+
+Delivered:
+- `WorkspaceBackendConfigFile` TOML schema with `server`, `data`, `limits`, and `runtimes.remote` sections.
+- Fixed workspace-local config path `.yoi/workspace-backend.local.toml`.
+- Defaults when config file is absent.
+- Unknown-field rejection.
+- Workspace-root relative path resolution and absolute path preservation.
+- Resolved config boundary for `ServerConfig`, listen address, and database path.
+- Data root derivation for `workspace.db` and `embedded-runtime`.
+- Secret value exclusion from schema; `token_ref` fails closed until secret ref resolution is implemented.
+- `yoi-workspace-server serve` loads config and applies only existing legacy dev CLI overrides.
+- Focused tests for defaults, unknown fields, path handling, data overrides, and secret field behavior.
+
+Validation:
+- `cargo test -p yoi-workspace-server`
+- `cargo check -p yoi`
+- `cargo fmt --check`
+- `git diff --check`
+- `nix build .#yoi --no-link`