feat: clean up secrets removed from the config with profile prune

[iamralch]

Removing a secret from config.yml left it behind: unload only unset configured secrets, so a removed variable stayed in the shell, a removed SSH key stayed in ssh-agent, and its cached value stayed in the keychain until profile clear wiped the whole profile.

• unload also unsets the secrets recorded as loaded and removes the SSH keys keysafe recorded adding, even if the config no longer names them. unload <NAME> accepts those names too; names keysafe never loaded still fail with "not found".

• profile prune <profile> (new) deletes cached secrets the config no longer names, under keysafe.<profile> and op-secrets-<profile>, and removes their SSH keys from ssh-agent. The profile stays loaded, so unload still knows the removed names. For a profile removed from the config, it deletes everything keysafe kept of it and forgets it; a name keysafe has nothing for still fails with "profile not found".

• doctor warns about orphaned keychain items, profiles recorded as loaded that the config no longer has, and orphaned keys ssh-agent still holds, each with the profile prune command that removes them.

• doctor's zsh-op warning now suggests profile prune instead of profile clear, and counts only the leftovers the config no longer names (configured ones move to keysafe.<profile> when read). With that, doctor only ever suggests prune; clear stays the "start over" command, and the help of each mentions the other.

load doesn't prune: we chose to report orphans in doctor and clean up only when asked.

Not covered:

• A profile unloaded before it was removed from the config has no record left, so doctor can't find its keychain items (the keychain is searched by exact service name). profile prune <name> still removes them.
• Variables already set in other open shells stay until unload runs there.
• The config validation message mentioned in the issue is left for a separate change.

Tested with unit tests for the cache, unload, profile prune and doctor (all 163 tests pass, clippy clean). Not yet tried against a real keychain and ssh-agent.

Closes #33