Add context timeouts for git and AI subprocesses

[JanSmrcka]

Problem

Git and commit-message subprocesses can block indefinitely under bad env/network/hooks.

Scope

• Introduce timeout wrapper using context.Context
• Apply to all git shell commands and AI commit-msg command
• Provide clear timeout/cancel user message

Acceptance Criteria

• 100% subprocess calls use timeout/cancelation path
• Timeout values are centralized/configurable
• Tests cover timeout behavior for at least one git and one AI path

Why

Timeouts improve reliability and prevent UI hangs.