Signals end every command, not only ssh to and ssh install (closes #48)

SIGINT, SIGTERM and SIGHUP were caught for the whole run, so the
mnemonic prompt waited for Enter and an interrupted `age encrypt -o`
put the encryption of the cut-off input in place. Now they end the
tool at once, except where a command cleans up first: `ssh to` and
`ssh install` while ssh or sftp runs, and `age encrypt -o` and
`age decrypt -o` while they write. A signal those two have received
when their input ends removes the unfinished file and exits 1; a later
one leaves the whole file in place. The tool stays on the main thread,
where Linux delivers the signal first. Signals the tool was started
ignoring stay ignored, so a run under nohup survives a hangup.

Model: opus-5-5

README.md

@@ -66,8 +66,9 @@ calls into `internal/`. The packages there are:
 - `internal/childmnemonic` derives a child mnemonic from the main one using
   BIP-85's own mnemonic application.
 - `internal/cli` builds the cobra command tree and runs it. Under it,
-  `cli/options` holds the flags every command shares, and `cli/ssh`, `cli/age`
-  and `cli/mnemonic` are the command groups.
+  `cli/options` holds the flags every command shares, `cli/signals` catches
+  SIGINT, SIGTERM and SIGHUP for the commands that clean up before they end, and
+  `cli/ssh`, `cli/age` and `cli/mnemonic` are the command groups.
 
 ### Adding a key type
 
@@ -288,6 +289,18 @@ girl mad pet galaxy egg matter matrix prison refuse sense ordinary nose
 Errors go to standard error and the exit status is 1, except for `ssh to`, which
 passes through `ssh`'s own exit status.
 
+SIGINT, SIGTERM and SIGHUP end any command at once, at the mnemonic prompt too,
+with the status a shell gives a program killed by that signal (130 for SIGINT).
+While `age encrypt -o` or `age decrypt -o` is writing the file, the signal makes
+it remove the unfinished file, leave a file already at the named path as it was,
+and exit with status 1. That holds for a signal that has reached `keyfunc` when
+its input ends; a later one leaves the whole file in place. Ctrl-C on a pipeline
+ends the input at the same moment, and on Linux `keyfunc` sees the signal first,
+though no system promises that. While `ssh to` or `ssh install` has `ssh` or
+`sftp` running, the signal ends that program instead, the tool removes its agent
+socket or working files, and it exits with status 1, or for `ssh to` with
+`ssh`'s own status if `ssh` reported one.
+
 ## Entrypoints
 
 The repo adheres to the