Every command ends on SIGINT, SIGTERM or SIGHUP; an interrupted age -o leaves no file (closes #48)

SIGINT, SIGTERM and SIGHUP are no longer caught for the whole run, so they end any command at once, the mnemonic prompt included. One package, internal/cli/signals, catches them only where cleanup is needed, and only those not ignored at start, so nohup still works: ssh to and ssh install while their child runs, and age encrypt -o and age decrypt -o while they write. A signal received by the time the input ends leaves no new file and exits 1; otherwise the whole file is put in place, never an unfinished one.

Judgement call: the guarantee is stated for a signal keyfunc has received, as Go cannot promise more; the main goroutine stays on the main thread so a Ctrl-C on a pipeline is seen first on Linux. Unverified on macOS.

Model: opus-5-5 (implementation); fable-5-1 (design)

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