chore(server): remove unused openshell-server-macros crate #4815

User Story

I maintain OpenShell's gateway authorization code. I need the unused openshell-server-macros crate removed so that contributors and agents don't treat it as the way RPC authorization is declared.

Description

crates/openshell-server-macros defines #[rpc_authz] / #[rpc_auth] proc macros that generated MethodAuth tables. Commit 9c019a9 ("Wire authorization into workspace model", RFC 0011 Phase 2) replaced them with per-method openshell.options.v1.authorization annotations in the proto, which crates/openshell-server/src/auth/descriptor_authz.rs reads at startup. Nothing depends on the macros crate any more. It only stays in the build because the workspace includes every crate under crates/*.

Remove the crate and the references that still describe it as current:

• crates/openshell-server-macros/ (the whole crate), plus the Cargo.lock update
• the crates/openshell-server-macros/ row in the AGENTS.md architecture table ("Compile-time helpers for gateway RPC authorization")
• the #[rpc_auth(...)] doc comment at crates/openshell-core/src/lib.rs:100-104, which should point at the proto authorization option instead

Context

• No crate lists openshell-server-macros as a dependency. A search outside the crate finds only the AGENTS.md row.
• Leaving the crate in place makes it compile, lint and cost CI time, and AGENTS.md (loaded into every agent session) points contributors at the wrong mechanism.
• Found while comparing gateway auth designs. It doesn't change behaviour.

Definition of Done

• crates/openshell-server-macros/ is deleted and Cargo.lock no longer lists it.
• AGENTS.md and crates/openshell-core/src/lib.rs no longer reference rpc_auth / openshell-server-macros. Any related skill or CONTRIBUTING.md text is updated per sync-agent-infra.
• mise run pre-commit (or the equivalent workspace build, clippy and test) passes.