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/integration.md

@@ -51,6 +51,7 @@ so `^prod` finds names starting with prod.
 | `--cached` | only what's cached, at once: nothing is loaded from Azure or Databricks |
 | `--max-age 2m` | load cached lists older than this again |
 | `--errors=false` | leave out the vaults and sources it couldn't read |
+| `--hidden` | list what's [hidden](repos.md#spring-boot-config) too, as `H` shows it (json: `"hidden": true`) |
 | `--color never` | no colours, for scripts |
 | `--icons=false` | no provider icons |
 | `--stars=false` | no ★ before starred names (in `favouriteColor`) |