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.

CLAUDE.md

@@ -179,7 +179,7 @@ read them à la carte.
 - **"How does the per-agent terminal classify + colour
   events?"** → [`docs/terminal-rendering.md`](docs/terminal-rendering.md).
 - **"How does claude get its prompt and what tools does it have?"** →
-  [`docs/turn-loop.md`](docs/turn-loop.md) (index: the loop, binary shape,
+  [`docs/turn-loop/`](docs/turn-loop/README.md) (index: the loop, binary shape,
   turn outcomes; sub-pages:
   [`claude-invocation`](docs/turn-loop/claude-invocation.md),
   [`config`](docs/turn-loop/config.md), [`mcp`](docs/turn-loop/mcp.md)).
@@ -239,7 +239,7 @@ The docs below own the details — this section just points at them.
 - **NixOS / nspawn quirks** (bind mounts, conf flags, etc.): →
   [`docs/gotchas.md`](docs/gotchas.md).
 - **Turn loop, sentinels (rate-limit, auth-failed), context
-  window:** → [`docs/turn-loop.md`](docs/turn-loop.md).
+  window:** → [`docs/turn-loop/`](docs/turn-loop/README.md).
 - **Two-step spawn, approval flow, flake.lock validation:** →
   [`docs/approvals.md`](docs/approvals.md).
 - **Pre-push lint hook** (catches tracker-tag and comment-block failures