docs: give turn-loop/ a README.md landing page

Part of hyperhive#1898 (b): every docs subdir should have a top-level
README.md link, achieved by moving/renaming where an existing file
already fits the role.

docs/turn-loop.md already served as the hub + index for the three
sub-pages under turn-loop/ (claude-invocation.md, config.md, mcp.md),
so it moves wholesale rather than leaving a redundant top-level
pointer stub. Fixes every inbound/relative link across the repo
(top-level README.md, CLAUDE.md, docs/persistence.md,
docs/tools/scheduling.md, the sub-pages own back-link, hive-agent
README + doc comments, hive-agent/Cargo.toml, .prettierignore per-file
exemption entry) - grepped the whole tree for both turn-loop.md and
turn-loop/ to find every reference rather than trusting a partial
list.

nix fmt clean, cargo check -p hive-agent clean.

hive-agent/src/main.rs

@@ -4,7 +4,7 @@
 //! generic and testable. Sibling: `hive-agent-mcp` (the MCP server this
 //! loop points claude at).
 //! Architecture lives in
-//! [`docs/turn-loop.md::Harness binary shape`](../../../docs/turn-loop.md).
+//! [`docs/turn-loop/::Harness binary shape`](../../../docs/turn-loop/README.md).
 //!
 //! Single bin crate: the module tree below (formerly this crate's `lib.rs`,
 //! before lib + bin were collapsed into one) plus the serve loop.
@@ -510,7 +510,7 @@ fn spawn_todo_socket(
 /// Boot — wires up the web UI, login state, stats, plugins, forge
 /// notifier, and either drops into `serve_loop` directly (`Online`) or
 /// parks on the login flow first (`NeedsLogin`). See
-/// `docs/turn-loop.md::Boot wiring`.
+/// `docs/turn-loop/README.md::Boot wiring`.
 async fn serve_main<S: Surface>(socket: &Path, poll_ms: u64) -> Result<()> {
     let port = std::env::var("HIVE_PORT")
         .ok()