Clean up the agent socket and working files when a signal ends the tool (closes #17)

Main ran the command tree on a background context, so SIGINT, SIGTERM or
SIGHUP killed the process before the deferred cleanup ran: the agent's
temporary directory and socket, and the install working directory, were
left behind.

Main now runs the tree on a signal.NotifyContext for those three signals.
A signal cancels the context, which ends the child ssh or sftp started
with exec.CommandContext, and the deferred cleanup then runs. The exit
status after a signal stays 1 unless ssh reported one of its own.

For "ssh to" the child is cancelled with SIGTERM rather than the default
kill, so ssh restores the terminal before it goes.

Model: opus-4-8

.golangci.yml

@@ -16,7 +16,6 @@ linters:
     - depguard         # Dependency allow/block lists
     - godot            # Requires comments to end with periods
     - wsl              # Deprecated, replaced by wsl_v5
-    - gomodguard       # Deprecated, replaced by gomodguard_v2
     - wrapcheck        # Too verbose for internal packages
     - varnamelen       # Short names like db, id are idiomatic Go
   settings:

README.md

@@ -308,6 +308,7 @@ The open issues that stand between the tree and a 1.0 release:
 
 - [#14 Choose a license and add LICENSE](https://git.eeqj.de/sneak/keyfunc/issues/14)
 - [#15 Decide the Go module path before 1.0](https://git.eeqj.de/sneak/keyfunc/issues/15)
+- [#22 1.0 release readiness](https://git.eeqj.de/sneak/keyfunc/issues/22)
 
 ## License