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.

.prettierignore

@@ -16,6 +16,6 @@ docs/persistence.md
 docs/terminal-rendering.md
 docs/tools/forge.md
 docs/tools/matrix.md
-docs/turn-loop.md
+docs/turn-loop/README.md
 docs/web-ui/agent.md
 docs/web-ui/dashboard.md