docs: repoint every reference the docs/ reorg left dangling

`docs/` was reorganised into topic subdirectories and the references to it
were not moved with it. Thirteen distinct paths and three relative links no
longer resolved, spread across nix, css, html, js, markdown and
.prettierignore — a stale pointer is not a markdown problem, it is wherever
someone wrote a path down.

Each mapping resolved to exactly one target. `docs/matrix.md` was the sole
ambiguous basename: .prettierignore lists `docs/tools/matrix.md` separately
and that entry still resolves, so the stale one is the integrations doc.

The three relative links were each one `../` too deep — from `<crate>/src/`
two levels reach the repo root. `hive-agent/src/login.rs` already had the
correct form, in the same crate, at the same depth.

.prettierignore is repointed rather than dropped, though nothing in the tree
runs prettier: no treefmt entry, no CI job, no package script. Whether that
config should exist at all is a separate question from whether it names
files that do.

nix/host-modules/swarm-otel.nix

@@ -1,7 +1,7 @@
 # The swarm's telemetry collector: one per swarm, in a `swarm-otel`
 # nixos-container beside the swarm's other shared services. This is the
 # **swarm** tier; `otel.nix` is the hive tier that forwards into it.
-# `docs/observability.md` owns the tier model and why the two stay separate
+# `docs/scheduler/observability.md` owns the tier model and why the two stay separate
 # when co-located.
 #
 # A container rather than a second host unit, for the same reason every