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.

frontend/packages/shared/src/terminal/terminal.css

@@ -14,7 +14,7 @@
    No `.term-input` here — composers are a separate concern, owned by
    each page's own CSS. Row taxonomy + layout contract documented in
    `docs/web-ui/shape.md::Shared terminal pane` and
-   `docs/terminal-rendering.md`. */
+   `docs/web-ui/terminal-rendering.md`. */
 
 .terminal-wrap {
   position: relative;
@@ -106,7 +106,7 @@
    is now four: the four severities every row already carries. Structural
    identity (this is a turn boundary, this is a tool call) is carried by
    the row's icon (`◆`, `🔧`, `✅`, …) and summary text instead of colour —
-   see docs/terminal-rendering.md. */
+   see docs/web-ui/terminal-rendering.md. */
 .live .level-debug { color: var(--muted); }
 .live .level-info  { color: var(--fg); }
 .live .level-warn  { color: var(--amber); border-left-color: var(--amber); }