Fix socket discovery splitting and quoting in ssh-find-agent.sh

[Copilot]

The script was relying on array command substitution for socket discovery, which triggers ShellCheck SC2207 and can mis-handle paths containing spaces. There were also a few tightly related quoting, scoping, and numeric-input issues in agent probing and selection.

• Socket discovery

  • Replace array command substitution with mapfile -t in sfa_find_all_agent_sockets()
  • Group find predicates so -type s applies to all supported socket name patterns
  • Preserve the existing socket filters while making path handling safe for whitespace

• Agent probing and selection

  • Fix the no-timeout branch to invoke ssh-add correctly
  • Quote SSH_AUTH_SOCK consistently when probing and exporting sockets
  • Localize temporary variables used during probing/menu rendering
  • Replace the fragile numeric-choice check with a regex-based validation

• Focused shell test coverage

  • Extend the existing shell test to create an agent socket under a directory with spaces
  • Verify ssh-find-agent -a resolves and exports that socket correctly

Example of the core change:

mapfile -t _ssh_agent_sockets < <(
  find "${sfa_path[@]}" -maxdepth 2 -type s \( \
    -name 'agent.*' \
    -o -name 'S.gpg-agent.ssh' \
    -o -name 'ssh' \
    -o -name 's.*.agent.*' \
    -o -regex '.*/ssh-.*/agent..*$' \
  \) 2>/dev/null |
    grep -E '/ssh-.*/agent.*|/gpg-.*/S.gpg-agent.ssh|/keyring-.*/ssh$|.*/ssh-.*/agent..*$|s\..*\.agent\..*'
)