Check the staged diff for credentials before push commits

`push` runs `git add --all .`, so anything lying in the project gets
committed, and with `mirror = true` it reaches a public server in the
same breath. It is the one action in mgsh that cannot be undone: a
deleted server repository comes back from an archive, a published
credential does not.

The staged diff is now scanned before the commit is made -- private keys,
GitHub/GitLab/Slack/AWS/PyPI tokens, and credential-shaped assignments --
and a hit is shown with file and line before asking whether to continue.
Declining leaves the changes staged but uncommitted, so removing the file
and adding a .gitignore entry is all it takes.

The hard part is not detection but silence. A scanner that cries wolf
gets answered with a reflexive "y" and stops being a safety net, so
values that are plainly environment references, dotted identifiers,
constant names, template slots or masked stand-ins are filtered out. A
test scans mgsh's own README and mgshrc.example -- both full of
credential-shaped text -- and fails if either would trip the check. It
caught the documentation for this very feature, which is why the README
describes the sample output instead of reproducing it.

For a line that legitimately looks like a credential there is
`mgsh:allow`, which suppresses that one line; `secretscan = off` turns
the check off entirely. Only an explicit "off" does that -- a typo in the
setting leaves the safety net in place, which is what the new falsy()
is for.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

mgshrc.example

@@ -42,6 +42,11 @@ gitpath     = /home/git
 #
 # mirror           = true         # `push` also mirrors via pushremote
 
+# --- safety ---
+# `push` checks the staged diff for private keys and API tokens before it
+# commits, and asks before continuing. Only an explicit "off" disables it.
+# secretscan       = off
+
 # --- per-project overrides ---
 # A <project>/.mgshrc overrides all of the above for that project only, except
 # base, gitname, gitemail and pushdefault, which stay global. Typical use: