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.

docs/turn-loop/claude-invocation.md

@@ -116,7 +116,7 @@ window with two triggers baked into its `run`:
   *still* overflows, `run` surfaces `Error::PromptTooLong`; `drive_turn`
   then archives the session (session lifecycle stays hive-side) and the
   serve loop requeues the message so it redelivers into a fresh session
-  (see [Turn outcomes](../turn-loop.md#turn-outcomes) — the wake prompt itself is tiny, so
+  (see [Turn outcomes](README.md#turn-outcomes) — the wake prompt itself is tiny, so
   the overflow was the accumulated context the archive clears).
 - **Proactive** — a turn finishes cleanly but the last inference's context
   size crossed the policy watermark. While the session is still healthy it