installer: print agent closing sections only for detected agents (#1606)

The closing summary of install.sh printed PI, GEMINI CLI, CODEX and
KIRO CLI sections on every machine, with 'X was not detected' nags for
agents the user does not have. Each of those sections now prints only
when the installer's existing detection found the agent (pi on PATH,
~/.gemini, codex_available, kiro_available). A detected-but-skipped
agent keeps its honest 'detected, skipped' section. OpenCode (no
detection leg) and Claude Code stay universal.

install.ps1 gates PI USERS and KIRO CLI USERS the same way; install.cmd
gates KIRO CLI USERS. Output only: nothing about what is installed,
detection, skip flags or exit codes changes.