docs(readme): document the curated shell aliases in §5 Day-to-day

List the on-by-default zsh aliases (navigation, git, nix, misc) in the
Day-to-day section so they're discoverable from the README, not just by
running `alias`. Note the doc location in the roadmap item.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

docs/ROADMAP.md

@@ -120,7 +120,8 @@ how to override it. Items marked ✓ are shipped.
   `nsearch`, `ngc`), plus `path` and `reload`. Scope decision: short where it
   helps but **never shadow a real binary** (same rule as rg/fd — the git block
   deliberately avoids `gs`/ghostscript); system/home rebuilds keep their full
-  `sys-update`/`home-update` names rather than getting cryptic aliases.
+  `sys-update`/`home-update` names rather than getting cryptic aliases. The set
+  is documented in README §5 (Day-to-day), and `alias` lists them in-shell.
 - **Theme-switch feedback:** ✓ the "rebuilding…" notification is now
   persistent (timeout 0) and replaced in place by "applied ✓" / failure
   via a synchronous tag, so a multi-minute switch never reads as a failed