[benbrandt]

Each protocol version's message types were reachable at two paths: agent_client_protocol::schema::v1::X and agent_client_protocol::v1::schema::X (same for v2). This PR keeps only the version modules. It was written as a stacked PR on #480; #480 has merged, so it targets main.

Changes

• v1::schema is now pub use agent_client_protocol_schema::v1 as schema;, and v2::schema is the same behind unstable_protocol_v2.
• schema::v1 and schema::v2 are removed. The root schema module now holds only the version-neutral items: ProtocolVersion, MaybeUndefined, IntoOption, IntoMaybeUndefined and SuccessorMessage. It keeps its name. The items in it are schema items, and renaming it would break every schema::ProtocolVersion import for no gain.
• Every crate, test, example, cookbook page, doctest and book page now uses the new paths:
  • Public doc examples import vN::schema and write schema::X.
  • Internal code uses a local vN::schema as vN alias where that keeps the diff small.
  • Rustdoc links that showed paths that don't exist (such as v2::NewSessionResponse in session/v2.rs) now show v2::schema::....
• The impl_proxy_jsonrpc! macro now reaches the proxy types through $crate::$ns::schema. The derive crate emits no schema paths.
• Docs:
  • md/migration-v5.md gets a rename table (schema::v1::X -> v1::schema::X, same for v2). The text that presented both paths is gone, and the "After" examples use the new paths.
  • The stability guidance in lib.rs and the README, plus md/design.md, md/protocol.md, md/protocol-v2.md and the "Adding a protocol version" checklist in md/versioning.md are updated.
• No compatibility aliases.

Left at the old path on purpose

• "Before" (3.6) code and text in md/migration-v5.md.
• The historical guides md/migration_v2.0.md and md/migration_v0.11.x.md, which describe the APIs of those releases.

Checks

These pass:

• cargo fmt --check
• both clippy runs
• cargo build --workspace --bins --examples --all-features --locked
• cargo hack check --feature-powerset --depth 2
• cargo test --workspace --all-features
• cargo test --no-default-features
• the rmcp_3 tests
• the nightly docs.rs build with -D warnings
• mdbook build
• typos

agent-client-protocol-derive --lib aborts under the local kache runner, which also happens on main.