feat: H shows what's hidden; Spring keys that don't look secret are hidden

Sources, files and secrets get a Hidden flag. The repo provider marks
Spring keys that match neither pattern, files with only those, and repos
with only such files, instead of leaving them out.

- Hidden items are left out of the panes, Go to and lazysecrets list;
  a pane's counter says how many it hides.
- H (lists.toggleHidden) shows them, dimmed whole in mutedColor;
  list --hidden lists them, json marks them "hidden".
- A revealed empty value shows "∅ no value (an empty string)", muted,
  instead of a blank line.

docs/keys.md

@@ -20,6 +20,7 @@ The rule: a letter in the lists, ctrl+ that letter where you type a filter
 | `[` / `]` | previous / next tab of the focused pane: Sources and Favourites in Sources; Secrets, Keys, Certs in Secrets |
 | `/` | filter the focused list by name or note ([how it matches](#searching)); `enter` keeps it, `esc` clears it. While typing: `ctrl+j`/`ctrl+k` move, `ctrl+y` copies, `ctrl+r` reveals, `ctrl+f` stars |
 | `` ` `` | switch the Sources pane between grouped by provider and a flat list; `enter`/`space` on a provider folds it |
+| `H` | show / hide what's hidden: [Spring keys](repos.md#spring-boot-config) that don't look secret, and files and repos with only those; shown dimmed |
 | `r` | refresh the focused pane |
 | `q`, `ctrl+c` | quit |