sshd: removing a key closes its connections #256

A removed SSH key keeps working on connections that were already open.

Authentication caches the key ID and scope in the connection's permissions. runExec (internal/sshd/sshd.go:299) reloads the user per exec but never the key; TouchSSHKey's result is ignored. With the CLI's ControlMaster, a connection carrying many sessions is the normal case, so a compromised key stays usable after keys remove for as long as its holder keeps the connection open.

Decision: revocation is immediate.

• Revalidate key existence, owner and current scope before every exec and every git transport session.
• sshd tracks open connections per key. keys remove, repo deploy-key remove and account disable close every connection on the affected keys, cancelling running commands, pushes included. An interrupted receive-pack updates no refs.
• An e2e test: open a multiplexed connection, remove the key, confirm the next exec is refused and a long-running command is cut off.