feat(providers): add Enclawed agent provider

[enclawed]

Summary

Adds Enclawed as an agent provider, in the same shape as Claude Code, Codex, Copilot, OpenCode. The actual sandbox image lives in a partner-owned external repo (enclawed/openshell-enclawed-sandbox) per the OpenClaw / NemoClaw pattern; this PR is only the OpenShell-side plumbing.

What's Enclawed?

A classification-gated AI agent gateway with an MCP-attested transport layer (see arXiv:2605.24248, source at github.com/enclawed/enclawed-oss). It composes admission control + tool-level authorization + a hash-chained audit log around standard MCP servers. The bundled secretary and codex apps run on top.

Why zero-credential discovery

Unlike the env-var-discovered providers, Enclawed bootstraps every credential into the operator's OS keyring at install time (libsecret on Linux, Keychain on macOS, Credential Manager on Windows) and reads none from the environment. The provider plugin has nothing to discover at provider-discovery time — modeled on GenericProvider for that reason. The matching sandbox image runs Enclawed's installer at first boot to populate the keyring inside the sandbox.

Changes

• providers/enclawed.yaml — declarative provider spec, category: agent, credentials: []. Endpoints declare the two backends Enclawed ships with: api.anthropic.com (codex / pi-coding-agent path) and 127.0.0.1:11434 (local Ollama; secretary path).
• crates/openshell-providers/src/providers/enclawed.rs — ProviderPlugin impl with empty discovery + smoke test.
• crates/openshell-providers/src/providers/mod.rs — module entry.
• crates/openshell-providers/src/lib.rs — registry entry.
• README.md — Supported Agents table row pointing at the partner sandbox repo.

Test plan

• mise run ci (lint + compile + tests) passes
• cargo test -p openshell-providers includes enclawed_provider_discovery_is_empty_by_default
• openshell provider create --type enclawed succeeds with no env credentials present
• Partner sandbox repo (enclawed/openshell-enclawed-sandbox) public and reachable from the README link

DCO signed-off-by Alfredo Metere. Commit is SSH-signed.

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