fix(install): require git alongside Node (pi install git: needs it)

Found in a clean cps sandbox run: a machine without git passes the Node check
but fails later inside 'pi install git:...' with an unclear error. Now checked
up front with an actionable message. CHANGELOG updated.

Decision on optionalDependencies (@modelcontextprotocol/sdk): KEEP. The heavy
install observed in the sandbox (~317 packages) is dominated by npm>=7
auto-installing the Pi peerDependencies stack, not the MCP SDK; removing the
SDK saves little and would make MCP enablement fragile across pi updates.

CHANGELOG.md

@@ -4,6 +4,10 @@
 
 ## [Unreleased]
 
+### Fixed
+- `install.sh` проверяет наличие `git` (нужен для `pi install git:...`) — на машине без git
+  установка падала позже с непонятной ошибкой. Найдено при прогоне в чистой cps-песочнице.
+
 ### Added
 - Security guardrails: `secret-scanner` (блок записи секретов), `commit-guard`
   (Conventional Commits + скан секретов в сообщении), `llm-redaction` (редакция